Auto-nominate PRs for beta backport

[Mark-Simulacrum]

We should nominate PRs for backport if any of the following are true:

• Description mentions a issue which is labeled as regression-from-stable-to-beta
• Description mentions a issue which is milestoned with the beta version number

[apiraino]

Tangential to this, I wonder if it would be possible to also figure out a logic to avoid automatically closing beta regressions.

People often manually re-open beta regressions (example) waiting for the backport to be approved/declined. Can we intercept somehow when GH closes an issue referenced by a PR that contains the words "Fixes/Closes #12345"?

I didn't investigate, just leaving this as a note ...