Further automate the creation of the stable branch

• Add a current_milestone task helper to automate the stable branch creation. This is effectively almost the same as the method used in scripts/tw-milestone-plan.rb
• Add a Makefile rule to run the task. This might be a bit overkill to have make run rake, but for the sake of not having two different tools invoked by folks running the docs release, I opted to add this.

Test locally

1. Check out this branch.

2. Run the make rule:

   make create-stable-branch

3. The stable branch 15.10 should be created and a new 15.10.Dockerfile should be committed to the branch.

Eventually the task will fail to complete, because the 15.10 branch is already present and a push is forbidden (protected branch). This is normal and expected for this test. In a real scenario, the task will successfully complete.

Here's what you'll see:

❯ make create-stable-branch


INFO: Creating stable branch...
bundle exec rake release:single
INFO: (gitlab-docs): Stashing local changes...
INFO: (gitlab-docs): Checking out main branch and pulling updates...
Switched to branch 'main'
From gitlab.com:gitlab-org/gitlab-docs
 * branch              main       -> FETCH_HEAD
INFO: (gitlab-docs): Creating branch 15.10...
Switched to a new branch '15.10'
INFO: (gitlab-docs): Creating file 15.10.Dockerfile...
INFO: (gitlab-docs): Adding file 15.10.Dockerfile and commiting changes to branch 15.10...
INFO: (gitlab-docs): Pushing branch 15.10. Don't create a merge request...
To gitlab.com:gitlab-org/gitlab-docs.git
 ! [rejected]          15.10 -> 15.10 (non-fast-forward)
error: failed to push some refs to 'gitlab.com:gitlab-org/gitlab-docs.git'
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. Integrate the remote changes (e.g.
hint: 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

And if you want to test this in the Ruby console:

❯ irb
irb(main):001:0> require_relative 'lib/tasks/task_helpers.rb'
=> true
irb(main):002:0> tasks = TaskHelpers.new
=> #<TaskHelpers:0x00007f6da986ea48>
irb(main):003:0> tasks.current_milestone
=> "15.10"