monitoring: fix CephPgImbalance alert rule expression

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>
(cherry picked from commit 5b4f7373655fa829af359d6e3cc61416964a97f0)

Conflicts:
	monitoring/ceph-mixin/prometheus_alerts.yml (remove cluster
label from alert since its not there in squid)
	monitoring/ceph-mixin/tests_alerts/test_alerts.yml (remove
cluster label from the alert since its not there in squid)