Fast and extensible multi-platform HTTP/1-2-3 web server with automatic HTTPS

ci: Run CI on PRs targeting minor version branches (#4164)

We decided that we'll use branches like `2.4` as the target for any changes that we might want to release in a `2.4.x` version like `2.4.1`, so that we can continue to merge changes targeting the next minor release (e.g. `2.5.0`) on master. Our CI config wasn't set up for this to work properly though, since it was only running checks on PRs targeting master. This should fix it. I couldn't find a way to do a pattern to only match digits for the branch names from Github's docs, it just looks like a pretty generic glob syntax. But this should do until we get to 3.0