ref(mergify): use the new configuration format and keys

[gustavovalverde]

Motivation

We tried using GitHub Merge queue without success, as this would have limited our actual automations, the use of multiple queues, and other flexibilities that Mergify gives us.

Additionally, Mergify already supports the Rulesets, allowing us to continue using their engine without having to change our actual GitHub configuration

Specifications & References

• https://docs.mergify.com/configuration/file-format/
• https://docs.mergify.com/workflow/actions/squash/
• https://docs.mergify.com/merge-protections/

Solution

• Transfer the pull_request_rules to the new priority_rules
• Use defaults for pull_request_rules and queue_rules
• Set the priority of high, medium and low, as required.

Tests

• This configuration was validated in https://dashboard.mergify.com/config-editor?repository=zebra&login=ZcashFoundation

Follow-up Work

• Revert edff643

PR Author's Checklist

• The PR name will make sense to users.
• The PR provides a CHANGELOG summary.
• The solution is tested.
• The documentation is up to date.
• The PR has a priority label.

PR Reviewer's Checklist

• The PR Author's checklist is complete.
• The PR resolves the issue.