Support filtering /in/ matched versions, not just filtering out

[michel-slm]

Release Monitoring currently supports 'Version filter ', but that is used for filtering out (e.g. filter out alpha;beta;rc)

There is no way to say 'I only want matches that match particular regexes' short of using a custom backend, since the custom regex is only available there and not in github, pypi, etc.

For example: there are multiple Django projects registered that tracked particular versions of Django; the latest of which is the one I just created for Django 6.x:

https://release-monitoring.org/project/388445/

Ideally I want to just use the PyPI backend, but then this will start matching against Django 7

[Zlopez]

You can still use version filter, it just filters out all the versions containing the string you put in there. And I assume this could help in the future #1540

[Conan-Kudo]

While that's true, it's not that helpful for tracking compatibility packages which need to track a specific version series. For example, once Django 7 is out, we'd get notifications for it for Django 6 packages too, which is not particularly helpful.