backport: limit what Depends-On changes are considered appropriate

Dependencies for a different branch are not normally relevant in the context of Mediawiki repositories.

The only exception being changes to operations/mediawiki-config. When this repo is targeted as a dependency, the only relevant branch is the production one (i.e. master). When this repo is the dependant, then we fall back to allowing all branches again.

Aslo refactored the validation code while maintaining semantics otherwise

Bug: T345304