ci: make v prefix optional in maintenance branch names

[dgw]

We aren't using the v prefix for maintenance branches. What we have are names like 7.1.x, 8.0.x, etc. and CI hasn't been running for 8.0 series maintenance patches like it should.

I already proposed this in the 8.0.2 release-prep PR (#2654), but it needs to be in the master branch too so future maintenance branches will get the new patterns automatically.

[dgw]

Note: We could probably remove the v prefix entirely. The distinction is supposed to be:

• *.*.x: maintenance branch
• v*.*.*: release tag

But I went with the smaller change for now. We're allowed to change the convention, and no release tag should ever actually have a name ending in .x, so in theory we could use v prefix everywhere.

[SnoopJ]

I approve of adjusting the workflow to match what's been done in the most recent release, making it optional is a nice middle-ground.

[dgw]

I figure there's no need to go through a whole new PR just to pull an added dev-requirements.txt comment line from #2654 upstream into master. I will get @SnoopJ's sign-off on the new comment via IRC (it was his idea).

Edit:

17:21:50 <+Sopel> [sopel] dgw commented on pull request #2655: I figure there's no need to go through a whole
new PR just to pull an added `dev-requirements.txt` comment line from #2654 upstream into `master`. I will
get @SnoopJ's sign-off on the new comment via IRC (it was his idea).
https://github.com/sopel-irc/sopel/pull/2655#issuecomment-2638257122
17:22:57 <+SnoopJ> :+1: 

then