Update runners owned by non-existing projects

What does this MR do and why?

This MR follows up on !176879 (merged) and adds a background migration to handle runners that are owned by deleted projects (referenced through sharding_key_id in ci_runners). There will be similar MRs to handle runner managers and runner taggings. If there is a fallback project association, we use the oldest still-existing association, otherwise we delete the runner.

Changelog: added

References

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• Handle sharding_key_id pointing to missing proj... (#512075 - closed)
• Update runner managers owned by non-existing pr... (!177423 - merged)
• Update runner taggings owned by non-existing pr... (!177453 - merged)

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