monitoring: fix CephPgImbalance alert rule expression
The alert CephPGImbalance doesn't take any device classes configured into account. As a result, there can be false positives when using mixed-size OSD disks.
Ref: https://github.com/rook/rook/discussions/13126#discussioncomment-
10043490
Fixes: https://tracker.ceph.com/issues/69690
Signed-off-by: Aashish Sharma <aasharma@redhat.com>