Workflow for introducing breaking changes

[barthalion]

Dumping it here for initial discussion.

When adding a potentially breaking change to the linter:

1. Add it to the deprecations (does not exist yet)
2. The deprecations field in the output should be a dict consisting of key: date mapping.
3. The date should be at least a month in the future at the time the change is merged.
4. To discuss: the linter could automatically bump the deprecation to an error based on the date, so we don't need to remember and merge this into multiple places.
5. Post-merge, the deprecation has to be announced at https://docs.flathub.org/blog/. The post should include what to change in the manifest/other files, and when it will become a blocking check.
6. Post-merge, we need to create issues for affected apps, and link back to the blog post. (TODO: figure out how to do this efficiently)