Merge backports with reduced backports label and no default MR

What does this MR do and why?

We sometimes have cases where a security implementation issue has ~"reduced backports", but specifically it is missing the MR targeting the default branch. Release-tools rejects merging the backports in this case meaning Release Managers need to manually merge the backports, creating release manager toil.

After asking the team on slack, the consensus was that this is likely technical debt. Release managers are generally pinged when any security issue has reduced backports to get an "ok" before applying the ~"reduced backports" label, so by the time merging is going to happen, release managers will have already verified whether or not it is safe to have them merged.

This MR adjusts the logic to allow automatic merging in this scenario.

Related to gitlab-com/gl-infra/delivery#19670 (closed)

Author Check-list

• [-] Has documentation been updated?