Handle 11.x branch alias changes

[catch56]

https://www.drupal.org/project/drupal/issues/3364646 introduces a branch alias for 11.x, which is likely to stay more or less the same when we switch to a proper 'main' branch too.

I think the implications for the branching script are:

1. When we branch 10.2.x, we probably don't need to change very much because it all says 10.2.x already, but we'd want to remove the branch alias.

2. But we'll want to also update the 11.x branch so the alias is for 10.3.x instead of 10.2.x