Teach rustbot about beta/stable PRs

[cuviper]

I wonder if we could improve messages like this on backports: rust-lang/rust#132468 (comment)

⚠️ Warning ⚠️

• Pull requests are usually filed against the master branch for this repo, but this one is against beta. Please double check that you specified the right target!

When the PR title starts with "[beta" or "[stable", we should instead warn if the PR is not targeting those specific branches.

(I left the strings open because some folks add a version in the brackets, like rust-lang/rust#132337)

[Urgau]

Looking at the recent backport PRs in rust-lang/rust I think #1860 successfully addressed this issue.

Is my assessment right @cuviper?

[cuviper]

Yeah, it's not harassing me any more, thanks! :)