Test rolling back 11.10.8 EE to 11.9.12 EE

A large customer has a significant performance regression with 11.10.8. While we work on fixing that issue, we want to test if it is possible to downgrade to 11.9.2.

Previously I tested something similar in a snippet: $1807309

I'll test this one in an issue. Even if this appears to work, this is not a supported option in general. This is just for informational purposes right now.

Particular considerations from @stanhu, who was working on this with the customer:

1. Can this be done in a no-downtime manner without downgrading migrations?
2. I see 1 background migrations in 11.10. I think it must have completed by now, but we may have to check.
3. We rolled out merge request approval groups in 11.10, I think (they mentioned this new feature). What happens to this if we downgrade?