mackup: deprecate

`mackup` works by moving config files to an
off-disk backup location (usually a cloud storage
provider like iCloud or Dropbox, etc.) and then
replacing the original file with a symlink to the
backed up version. The intended use is to make
sure program configuration settings and
preferences can be restored if you need to replace
computers, factory reset, etc.

As documented
[here](lra/mackup#2035),
[here](lra/mackup#1924),
[here](lra/mackup#1969 (reply in thread)),
`mackup backup` no longer works on MacOS Sonoma
(likely due to [security
changes](lra/mackup#1924 (comment))
in the app sandbox in Sonoma) and running either
the backup or restore commands on MacOS Sonoma
will result in the loss of all preference files.
Users who backup to a storage provider that has
versioning or backup snapshots may be able to
restore preferences to an older version. For users
backing up to other storage providers that do not
have this feature, such as iCloud, the command
results in total data loss. This issue has been
known to the developer for over a year but there
is no fix, no mention of the issue in the project
README, no warning when running the command, and
no indication in the program output that the
backup process may not have completed properly -
neither in the dry-run mode nor in live mode.

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1 @@
+# Please fill out one of the templates on: https://github.com/Homebrew/homebrew-core/issues/new/choose or we will close it without comment.

.github/ISSUE_TEMPLATE/bug.yml

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+name: New issue for Reproducible Bug
+description: "If you're sure it's reproducible and not just your machine: submit an issue so we can investigate."
+labels: [bug]
+body:
+  - type: markdown
+    attributes:
+      value: Please note we will close your issue without comment if you do not fill out the issue checklist below and provide ALL the requested information. If you repeatedly fail to use the issue template, we will block you from ever submitting issues to Homebrew again.
+  - type: textarea
+    attributes:
+      render: shell
+      label: "`brew gist-logs <formula>` link OR `brew config` AND `brew doctor` output"
+    validations:
+      required: true
+  - type: checkboxes
+    attributes:
+      label: Verification
+      description: Please verify that you've followed these steps. If you cannot truthfully check these boxes, open a discussion at https://github.com/orgs/Homebrew/discussions instead.
+      options:
+        - label: My `brew doctor` output says `Your system is ready to brew.` and am still able to reproduce my issue.
+          required: true
+        - label: I ran `brew update` and am still able to reproduce my issue.
+          required: true
+        - label: I have resolved all warnings from `brew doctor` and that did not fix my problem.
+          required: true
+        - label: I searched for recent similar issues at https://github.com/Homebrew/homebrew-core/issues?q=is%3Aissue and found no duplicates.
+          required: true
+  - type: textarea
+    attributes:
+      label: What were you trying to do (and why)?
+    validations:
+      required: true
+  - type: textarea
+    attributes:
+      label: What happened (include all command output)?
+    validations:
+      required: true
+  - type: textarea
+    attributes:
+      label: What did you expect to happen?
+    validations:
+      required: true
+  - type: textarea
+    attributes:
+      render: shell
+      label: Step-by-step reproduction instructions (by running `brew` commands)
+    validations:
+      required: true

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+
+contact_links:
+  - name: Request a change to a formula
+    url: https://docs.brew.sh/How-To-Open-a-Homebrew-Pull-Request
+    about: Is something missing in a formula? Follow this guide to request new features in formulae.
+  - name: Get help in GitHub Discussions
+    url: https://github.com/orgs/Homebrew/discussions
+    about: Have a question? Not sure if your issue affects everyone reproducibly? The quickest way to get help is on Homebrew's GitHub Discussions!
+  - name: New issue on Homebrew/brew
+    url: https://github.com/Homebrew/brew/issues/new/choose
+    about: Having a `brew` problem that's not from a `brew install` or `brew upgrade` of a single formula/package? Report it to Homebrew/brew (the Homebrew package manager).
+  - name: New issue on Homebrew/homebrew-cask
+    url: https://github.com/Homebrew/homebrew-cask/issues/new/choose
+    about: Having a Homebrew Cask problem? Report it to Homebrew/homebrew-cask (the cask tap/repository)

.github/PULL_REQUEST_TEMPLATE.md

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+<!-- Use [x] to mark item done, or just click the checkboxes with device pointer -->
+
+- [ ] Have you followed the [guidelines for contributing](https://github.com/Homebrew/homebrew-core/blob/HEAD/CONTRIBUTING.md)?
+- [ ] Have you ensured that your commits follow the [commit style guide](https://docs.brew.sh/Formula-Cookbook#commit)?
+- [ ] Have you checked that there aren't other open [pull requests](https://github.com/Homebrew/homebrew-core/pulls) for the same formula update/change?
+- [ ] Have you built your formula locally with `HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>`, where `<formula>` is the name of the formula you're submitting?
+- [ ] Is your test running fine `brew test <formula>`, where `<formula>` is the name of the formula you're submitting?
+- [ ] Does your build pass `brew audit --strict <formula>` (after doing `HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>`)? If this is a new formula, does it pass `brew audit --new <formula>`?
+
+-----

.github/actionlint.yaml

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+self-hosted-runner:
+  # Labels of self-hosted runner in array of strings.
+  labels: []
+# Configuration variables in array of strings defined in your repository or
+# organization. `null` means disabling configuration variables check.
+# Empty array means no configuration variable is allowed.
+config-variables: []