Automatically tag PRs with a milestone

It would be nice if we can tag PRs with the version of CATcher that includes those changes.
This can be done manually by tagging a milestone to each PR.

We could try automating this as follows:
1. Create a GH actions' workflow that tags PRs **upon merge** with a milestone named "Next release"
2. When we release a new version, we rename the milestone using the actual version number that was chosen (e.g. V3.4.0)
3. Lastly, we create another "Next release" milestone for upcoming releases

We can explore if Step 2 and 3 can be automated by GH actions, as well.

No files or tests are named. Start by reviewing the repository's existing GitHub Actions workflows and release process, then determine how merged PRs and milestone renaming are currently handled. Done means merged PRs receive the "Next release" milestone and releases rename the prior milestone while creating a new one, with any feasible automation covered.

Written by the indexing model from the issue text.