Run mysql ce-ee upgrade on stable branches

What does this MR do?

This MR changes upgrade-mysql-ce-to-ee job to be run only on stable branches.

What was considered in https://gitlab.com/gitlab-org/gitlab-ee/issues/12327:

• Changing the job to pull from a stable branch instead of master when no CE matching branch has been found can be done but requires a number of changes. We currently have no way of getting the target branch without using pipelines for merge requests https://gitlab.com/gitlab-org/gitlab-ce/issues/57190 which is not working. This would require a lot of guessing which would be error prone.

This job will only run on stable branches because ee-specific-check scripts will always find the matching CE branch and will always be able to execute the upgrade.

This job will not run on feature branches unless there is a same branch in CE which can't always be the case (nor we can require it additionally without complicating things even further).

This should give us at least some confidence with running mysql ce-ee upgrades in stable branches.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/12327