Allow restricting “supportable” branches

In #2189131: Update DrupalorgVersioncontrolLabelVersionMapperGit for semantic versioning in contrib, there was an issue raised with the update status client implications for our planned support for both {major}.x-dev and {major}.{minor}.x-dev development releases. If Drupal.org reports different combinations of the {major}. and {major}.{minor}. release series being supported, we have twice as much to consider. It isn’t worth it when {major}. will only ever be that dev release, no tagged releases.

My thinking is that we should never allow that branch to be marked as supported by the maintainer. I’m making this a slightly-flexible configuration in project release, rather than trying to get it in via various alter hooks.