Memcached used as RateLimiter backend is no-op

## Summary: When Memcached is used as backend: https://github.com/nextcloud/server/blob/c705b8fcb3de7910e67cd2ed2d2b38653f58962a/lib/private/Server.php#L787-L799 The following code block is problematic: https://github.com/nextcloud/server/blob/90104bc1c448c6da2fd3e052fca75bb3fb261c87/lib/private/Memcache/Memcached.php#L135-L139 I guess we need to check the actual cache type and use the DB backend when Memcached is used? ## Impact Any action that partly resets any cache entry will wipe rate limit attempts and future bruteforce protection (with https://github.com/nextcloud/server/pull/39870 )