Allow Security MRs targeting the default branch to be considered for `Pick into X.Y` cherry-picking
This supersedes &297 and would allow Security MRs targeting master
to have Pick into X.Y
labels applied, completely negating the need for developers to create backports.
We may want additional verification around this so that when the label is applied, we check to make sure it would cherry-pick either to the preparation branch or the stable branch. If it doesn't, remove the label and issue a warning to the developer that they need a manual backport.
Edited by Robert Speicher