Attempt to migrate ci_runner_taggings table (try 2)

What does this MR do and why?

!186236 (merged) attempted to ensure that the taggings data is copied to ci_runner_taggings due to a background migration that did not work properly. However, this migration only ran if the ci_runner_taggings were empty.

If a user upgraded to GitLab 17.8.0 - 17.8.6 and added new CI runners, then the ci_runner_taggings table would not empty.

This migration attempts to relax the constraints in !186236 (merged) by performing the migration if the ci_runner_taggings table has < 50% of the entries of the runner tags from the taggings table.

This might have an unintended side effect of adding tags that were removed, but that is generally better than the alternative of missing tags altogether.

Relates to #524402 (closed)

Changelog: fixed

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