turn LDAP's treat remnants as disabled feature config-independent

[blizzz]

Summary

• [x] requires https://github.com/nextcloud/server/pull/46991

Since former LDAP users cannot be bound to a server configuration anymore (or theoretically belong to more than one) this feature has to be config independent. This PR:

• changes the existing logic from config-bound to app-global
• including web interface changes
• has a migration step to set the new values based on the old ones – one previous activation enables it for the whole LDAP backend

A backport to 28 will need further adjustments as IAppConfig was only introduced in 29.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• [ ] Tests (unit, integration, api and/or acceptance) are included
• [X] Screenshots before/after for front-end changes
• [ ] Documentation (manuals or wiki) has been updated or is not required
• [ ] Backports requested where applicable (ex: critical bugfixes)