Convert ci_builds-runner_id FK to LFK

What does this MR do and why?

Related to #356153

This MR deletes the foreign key of ci_builds#runner_id-ci_runners and adds a loose foreign key definition for it.

The migration and the code changes are generated based on the scripts/decomposition/generate-loose-foreign-key script: https://docs.gitlab.com/ee/development/database/loose_foreign_keys#the-scriptsdecompositiongenerate-loose-foreign-key

Background

Because of the foreign key constraint between Ci::Build and Ci::Runner, every time a runner is deleted the builds that were executed by this runner would need to be updated to have the runner_id value nullified.

When busy runners are deleted it causes massive spikes in number of updates to the ci_builds table:

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