Run CI for commits and PRs to release branches (#5475)
Run CI for commits and PRs to release branches (#5475)

In normal operation, Boulder does not have release branches, only
release tags. However, when we need to add hotfix commits on top of an
old release, we create a release branch, merge the commits there, and
then produce a new tag pointing at the tip of that branch. These release
branches are documented[1] to be named `refs/heads/release-branch-*`.
Therefore, we should run CI for PRs targeting, and new commits on, those
release branches.

[1] https://github.com/letsencrypt/boulder-release-process#when-main-is-dirty
jcjones/boulder,letsencrypt/boulder,letsencrypt/boulder,jsha/boulder,letsencrypt/boulder,jcjones/boulder,jsha/boulder,jcjones/boulder,jsha/boulder,jcjones/boulder