Skip tests for migrations older than 3 milestones

What does this MR do and why?

Related to #473169 (comment 2006076275).

Once we execute migration there is not much benefit from executing its test over and over again until migrations are squashed and tests removed. We can also more regularly remove just tests (example) which is easier than a migration squash, bit it will require manual step, at least to merge the MR even if it's created automatically by Housekeeper.

With this change all tests for migrations older than 3 milestones will be automatically skipped, which should decrease job execution time.

We can still pursue regular test cleanup.

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