Switch usage of tag_list to job definitions

Problem

In Deduplicate tag_list in Ci::JobDefinition (#565405 - closed) (Phase 2) we started storing tag_list in job definitions. In Phase 3, we will migrate existing job tags into the job definitions table.

At this stage we are still using the legacy tables for tag list. We need to use the job definition as SSoT.

Proposal

• Change tag_list to read from the job definition. If job definition does not exist, fall back to using tags.
• Deprecate tag_list= because we cannot update job definitions. This is an attr writer used when creating the records but it's not updated after job creation.
• Ensure that any features related to tag list (e.g. job queue scheduling) make use of the job definition data.