Backports: identify unlinked regressions and pull requests

[apiraino]

From a recent meeting

• Detect P-{critical,high} closed issues but aren't assigned a team label (ie, they would fail to be looked at by any team for backport). Example: we have an open PR that fixes a P-{critical,high} issue but the PR isn't tagged with a team label and thus would not be considered for backport
• A P-{critical,high} issue but the PR is still open and is S-waiting-on-review? We could try to expedite reviews for those PRs or at least have some visibility into them in the triage meeting
• We should identify the team that should own them and fix the label
• Do not autonominate for backport PRs fixing P-{low,medium} regressions (by definition they're not high priority)