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.

roles/gitlab/tasks/main.yml

@@ -45,6 +45,7 @@
         gitlab_rails['lfs_enabled'] = true
         gitlab_rails['gitlab_shell_ssh_port'] = 222
         gitlab_rails['gitlab_default_can_create_group'] = false
+        gitlab_rails['gitlab_username_changing_enabled'] = false
         gitlab_rails['initial_root_password'] = "{{ vault_gitlab_root_password }}"
         gitlab_rails['smtp_enable'] = true
         gitlab_rails['smtp_address'] = 'mail.archlinux.org'