Hide inactive approvers

What does this MR do and why?

When a user becomes inactive (e.g. blocked, banned, deactivated), they can't approve a MR. But as of this writing, we still show blocked users when they're added as approvers to an approval rule.

To prevent that, we filter inactive users from the list of approvers.

How to set up and validate locally

1. Create an approval rule with multiple users (User A and User B) and group approvers (User C and User D as members).
2. Create a MR.
3. Block User B and User D.
4. View the MR approval rules and User B and User D shouldn't show up as eligible approvers.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #10477 (closed)