[CI/CD] Refactor backports flow so that it can determine automatically where...

[CI/CD] Refactor backports flow so that it can determine automatically where to do a backport based on labels (#7976)

This PR changes the command-backport.yml flow so that the branch names
are not hardcoded in the pipeline file but will be parsed from the
labels.
The idea is that there are going to be a label representing each stable
branch:
- A4-backport-stable2407 
- A4-backport-stable2409 
- A4-backport-stable2412 
- A4-backport-stable2503

If the backport is needed to any of those branches or to all of them,
the corresponding label can be set on the PR and the branch name will be
parsed from it.
The labels need to be created in the repo and adjusted as soon as there
is a new release appears or an old one disappears.