fix(dev-infra): ensure PRs targeting a specific branch are not merged into other branches

[gkalpak]

In pr/merge/defaults/labels.ts > getDefaultTargetLabelConfiguration(), there should be a check to ensure that we don't try to merge a PR to latest/rc/next branches if the PR is targeting a specific branch (for example, here).

This was originally discussed in angular/angular#38656 (comment).

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