Following the discussion in #t-infra/bootstrap > ✔ Have triagebot create backport nomination threads? where we noticed that bootstrap backport nominations were difficult to track, since:

• We don't really have a dedicated person / group of people checking for bootstrap backport nominations.
• We don't really have an establish way of handling bootstrap backports.

This PR configures triagebot to open backport nominations threads (indicated for bootstrap team if a given PR has the suitable team label, T-bootstrap Relevant to the bootstrap subteam: Rust's build system (x.py and src/bootstrap) ) in #t-infra/bootstrap/backports. The current configuration is such that only members subscribed to this backport channel will be pinged, instead of unconditionally pinging the whole team.

TODOs and Unresolved Questions

• Ping members who opt-in subscribe to backport channel, or ping the whole bootstrap team?
• (Best for follow-up) Maybe also document bootstrap team's backport process / decision process in Forge? It's a bit different from compiler team's backport process because we don't have weekly bootstrap triage meetings and thus don't have sync backport decisions.