Remove semver usage

⚠ This MR is based on the pedropombeiro/358406/2-accept-patch-upgrades-from-newer-major-releases branch, please change to master before merging ⚠

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR removes the usage of the recently introduced (%15.1) semver field in Ci::Runner, as we decided to change the approach. Therefore we're stopping the BackfillCiRunnerSemver background migration which was introduced in %15.2 (therefore executed only in GitLab.com) by following the recommended approach in https://docs.gitlab.com/ee/development/database/deleting_migrations.html. The semver field and related indices will be removed in a future milestone.

MRs being reverted: !89024 (merged), !89054 (merged)

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