LDAP authentication: `admin_user` tried via LDAP first

### What happened? After switching from `bind_dn` & `bind_password` usage to Single Bind (i.e. no `bind_password`), API calls via `security.admin_user` & `security.admin_password` started to authenticate against LDAP first, then against the internal user database. Every API call now triggers a `Cannot bind user ...` error in `grafana.log` and takes 3 seconds to finish. ### What did you expect to happen? Grafana should ask its internal user database first, at least for `admin_user`. ### Did this work before? Unknown. ### How do we reproduce it? Use Single Bind and call `/api/orgs` with `admin_user` & `admin_password` credentials. ### Is the bug inside a dashboard panel? No. ### Environment (with versions)? Grafana: 11.1.2 OS: Debian 12 Browser: curl ### Grafana platform? A package manager (APT, YUM, BREW, etc.) ### Datasource(s)? _No response_