nixos/gitlab: ensure service started again after dependency restarts (#245240)

When a dependency, like postgresql.service or redis-gitlab.service, had
been stopped and started at switch-to-configuration time, gitlab.service
and its helper units had been stopped but not started again.
`multi-user.target` only has a `Wants` relation to gitlab.target, but
once gitlab.target has been successfully started once and is not stopped/
restarted again, it does not cause all its dependencies to stay activated
the whole time.

This commit fixes this by upgrading the dependy relationship of
gitlab.service towards gitlab.target from a "Wants" to a "Requires". It
should be enough to do this for this single unit part of gitlab.target
only, as all other units wantedBy gitlab.target are pulled in by
gitlab.service as well or have bindsTo relations.

nixos/modules/services/misc/gitlab.nix

@@ -1635,7 +1635,7 @@ in {
         "gitlab-config.service"
         "gitlab-db-config.service"
       ] ++ optional (cfg.databaseHost == "") "postgresql.service";
-      wantedBy = [ "gitlab.target" ];
+      requiredBy = [ "gitlab.target" ];
       partOf = [ "gitlab.target" ];
       environment = gitlabEnv;
       path = with pkgs; [