Use optional encryption strategy for Runners Tokens

What does this MR do?

This cleans runner tokens migration, by following that plan:

• Steal migration to ensure that all rows are migrated,
• Search via token and encrypted, allow forcing search via encrypted only,
• Make encrypted: to be closure, to control it via feature-flag,
• Verify the behavior of fallback on GitLab.com after disabling it (after this MR).

What are the relevant issue numbers?

Resolves https://gitlab.com/gitlab-org/gitlab-ce/issues/54859

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer