Ignore the column token in ci_runners

Original Issue: Remove unencrypted runner tokens from database (#410315 - closed)

• Migrated the plain token encrypted in %18.2 in this MR. maintenance: Encrypt missed ci_runner plain tokens (!196265 - merged)
• Finalize the BBM in this issue #553061 (closed) - this will be done in %18.3 - Doc Reference: https://docs.gitlab.com/development/database/batched_background_migrations/#finalize-a-batched-background-migration
• Implementation Plan:
  • Make the token encryption required (optional now) in the runner model, and it won't be referenced in the Rails code. REF
  • And add the `ignore_column :token, remove_with: '18.5', remove_after: '2025-10-22' Reference: https://docs.gitlab.com/development/database/avoiding_downtime_in_migrations/#ignoring-the-column-release-m