Remove sharding_key_id usages from runner-related tables

Once the sharding key has been changed to organization_id, we can stop using sharding_key_id in runner-related tables. The column will be dropped in the subsequent milestone.

• App code: Ci::Runner#ensure_manager, Ci::Runner sharding_key_id validations, Ci::RunnerManager sharding_key_id validations, Ci::RunnerTagging sharding_key_id validation, Ci::Runners::RegisterRunnerService#attrs_from_token.
• LFKs: Some LFKs exist on sharding_key_id so that a runner tagging is removed if a project or group is removed. This is not be possible anymore with only the organization_id.