Fix Ci::RunnerTagging sharding_key_id validation

What does this MR do and why?

This MR fixes Ci::RunnerTagging sharding_key_id validation so that it doesn't force a value on instance type runners.

Changelog: fixed

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• Fix Ci::RunnerTagging sharding_key_id validatio... (#519245 - closed)
• Create ci_runner_taggings table (!170778 - merged)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.