Is your feature request related to a problem? Please describe.

Currently, the auto milestone script incorrectly applies the milestone of the alpha version for PRs based on beta or rc. See, for example, #17219, which was merged to beta, but had the 2025.1 milestone auto-applied, even though it was going in to the 2024.4 release.

Added:
It would be nice to be able to specify the beta milestone when sending out notifications.
https://github.com/nvaccess/nvda/pull/17590/files#r1984622052

Describe the solution you'd like

Have separate milestones for master and beta merges, and use the appropriate milestone when merging a pull request based on the base branch.

Consider extending this to rc, though this should be less necessary, as merges to rc are rare.

Describe alternatives you've considered

• Make no change, but make sure we are vigilant about correcting auto milestones.
• Only apply auto milestones to merges to master.

Additional context

Milestones are automatically applied to issues that are closed as completed. We would need to make sure the correct milestone is applied to these as well, by inspecting the PR that closes them. I am unsure if this is possible with the GitHub Actions API.