inGroup and getUserGroups have different cache keys and implementations

[PVince81]

I'd expect Group_LDAP::inGroup to simply use Group_LDAP::getUserGroups or at least use the same cache keys.

This means that it could happen that the cache contains slightly different values, leading to bugs like https://github.com/owncloud/core/issues/26683 where one API which uses getUserGroups() sees that the user is member of a group, but inGroup says the user isn't.

@jvillafanez @owncloud/ldap