MR approvals with password don't work when LDAP user changes its username in GitLab

Summary

In a GitLab setup with LDAP authentication, a user changing its username causes the MR approval with password to fail with wrong password

Steps to reproduce

1. Setup a GitLab with LDAP authentication
2. Setup a Project with required authentication when approving a merge request
3. Login as a LDAP user (say, "fbar")
4. Change its username under Settings > Account (say "foo.bar")
5. Try to approve an MR in the project above

What is the current bug behavior?

Authentication error

What is the expected correct behavior?

Authentication succeeds

Possible fixes

We've narrowed it down to [this line of code[(https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/services/merge_requests/approval_service.rb#L34). Apparently it doesn't take the case when the LDAP username is different than GitLab's.

ZD https://gitlab.zendesk.com/agent/tickets/152008 (internal)