Add more label checks to dangerfile.py

Fail for backports with "Affects v9.x" labels set

Backports are not expected to have any "Affects v9.x" labels set since those are only meant to be set for merge requests that should have backports created for them.

Fail for branches using old-style version suffixes

Using "-v9_x" and "-v9.x" version suffixes for branch names is now deprecated since some automation logic does not handle these. Fail for any merge request using such old-style version suffixes.

Warn about auto-generated merge request titles

Merge request titles auto-generated by GitLab are often a source of confusion regarding the actual contents of a given merge request. Warn for merge requests containing titles that look like auto-generated ones.

Fail for merge requests with "Affects v9.x" labels

Setting "Affects v9.x" labels on a merge request duplicates information already present on the GitLab issue associated with that merge request. For trivial merge requests that are not associated with any GitLab issue, setting the "Affects v9.x" label(s) is considered unnecessary. Trigger a failure for every merge request marked with at least one "Affects v9.x" label.

Closes #4706 (closed), #4707 (closed)