Draft: Drop the auto_assign_gitlab_duo_pro_seats NamespaceSetting column

What does this MR do and why?

❗ Do not merge this MR before %18.4 in order to respect the database column dropping process ❗

Drop the enable_auto_assign_gitlab_duo_pro_seats setting from the namespace_settings table. This is part of rolling back the changes from Iteration 1.0, Automatic Duo pro assignment via... (&13637) by reverting Add enable_auto_assign_gitlab_duo_pro_seats nam... (!150824 - merged).

This MR is preceded by Revert automatic Duo seat assignment changes be... (!195161 - merged) which cleans up the business logic from &13637, as well as Ignore the enable_auto_assign_gitlab_duo_pro_se... (!197454 - merged) for ignoring the column to be dropped.

After this MR, one last MR will be needed to clean up one last reference in this spec. This has to be done after this MR is merged in order for the post-migration added here to have time to remove the enable_auto_assign_gitlab_duo_pro_seats column.

References

• Relevant issue: https://gitlab.com/gitlab-org/gitlab/-/issues/548750+.
• Parent MRs: Revert automatic Duo seat assignment changes be... (!195161 - merged) and Ignore the enable_auto_assign_gitlab_duo_pro_se... (!197454 - merged).

Screenshots or screen recordings

No visual changes expected.

How to set up and validate locally

CI pipeline should continue to pass. You can also check through rails dbconsole that \d namespace_settings no longer has an enable_auto_assign_gitlab_duo_pro_seats column.

MR acceptance checklist

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

Post-deployment steps

• Clean up the auto_assign_gitlab_duo_pro_seats feature flag through Chatops (as per this documentation)