Disallow changing of the Gitlab username

As keycloak is the canonical source of truth and interacting with gitlab usually requires the username as identifier. As we don't want to accidently assign permissions to a changed username disallow changing of usernames altogether in Gitlab.

approved this merge request

hm https://gitlab.com/gitlab-org/gitlab/-/issues/21794

approved this merge request

We should roll this out and verify its really really really gone -- according to the linked issue

More interesting references, some of which fixed i assume: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/23034 https://gitlab.com/gitlab-org/gitlab/-/merge_requests/21987 https://gitlab.com/gitlab-org/gitlab/-/issues/24605

mentioned in commit 2231fad0

merged

Deployed on our server, where it indeed hides the account rename option, so it seems we are good to go. But we do need to keep this mind and check again when we deploy gluebuddy.