Support releases from older release lines in modernized repos #573

Context

Modernized repos release with python-semantic-release from main only. Pre-modernization repos published on tag push or GitHub Release, which works from any branch. This means patching an older release line now needs a manual twine upload with PyPI rights, which is not a good approach and needs rights.

tutor-contrib-aspects#1330 restores this with a manual workflow_dispatch run and PSR multibranch releases. The proposal to adopt it everywhere is in this comment.

Scope

• Restrict the maintenance branch match to the org's branch pattern, for example release/ulmo (^release/[a-z]+$), or optionally 1.x (^\d+.x$).
• Add the change to the sample-plugin release workflow.
• Include it in future modernization PRs under public-engineering#506.
• Backport it to already modernized repos.
• Verify one end-to-end release from an older release line.

Notes

• release/<name> is the current org convention for release lines (108 branches), while N.x branches are rare.
• A manual run uses the workflow file from the selected branch, so backports only help release branches created afterwards.