[Backport M66] chore/build: Teach JetBrains push-git-tag-for-next-release.sh about release branches

[sourcegraph-release-bot]

These scripts predate release branches and used to always look at global tags. This change makes it work on release branches. It uses git to list the commits on the release branch, then does the same version bump calculation using the branch's tags.

It takes care to handle newly budding branches which are just a commit on main that has no backports yet. In that case it looks at all of the available tags.

It will work carrying a branch from prerelease through to release:

# At the start of the branch
$ ./scripts/push-git-tag-for-next-release.sh --minor --nightly
# During prerelease, assume we have accumulated some commits on the branch now:
$ ./scripts/push-git-tag-for-next-release.sh --patch --nightly
# Transitioning to release:
$ ./scripts/push-git-tag-for-next-release.sh --minor
# Hopefully never needed, but making emergency patches on stable:
$ ./scripts/push-git-tag-for-next-release.sh --patch

There are some caveats with this tool:

• When working on a release branch, it only has local view of that branch's tags. Misusing it should be harmless because creating a tag which conflicts with an existing tag will fail when the tag is pushed.
• It does not enforce the new practice of stable builds having even version numbers and prerelease builds having odd version numbers.

If you ever need to work around this tool, just author and push git tags yourself. To this point release captains have been doing that anyway.

Adds a --dry-run option which makes testing easier. Removes outdated docs and points to Notion instead.

Test plan

# Pretend you're just cooking up a release branch
$ git remote update origin
$ git checkout -b my-release-branch origin/main
$ cd jetbrains
$ ./scripts/push-git-tag-for-next-release.sh --minor --nightly --dry-run
# It should suggest a minor version bump based on all the version tags in the repo
...
# Pretend you're on a release branch, we're doing a patch release
$ git fetch origin refs/heads/jb-v7.8.x
$ git checkout FETCH_HEAD
$ git checkout <this PR> -- scripts/next-release.sh scripts/push-git-tag-for-next-release.sh
$ git commit -m 'test test test do not push'  # script insists the repo is clean
$ ./scripts/push-git-tag-for-next-release.sh --minor --dry-run
# It should suggest v7.8.1

I am the release captain and will dogfood this script in the current release.
Backport b912145 from #6881