mgr/DaemonServer: clarify ok-to-upgrade error message for CRUSH buckets

Refine the error string in DaemonServer.cc returned by the
ok-to-upgrade command when OSDs in a CRUSH bucket cannot be upgraded.

The original message is ambiguous. It fails to clearly convey that
stopping *any* individual OSD in that specific bucket will drop PGs
offline, meaning no OSDs within that bucket can be safely upgraded at
this time.

Update the phrasing to explicitly state that at least X PGs will go offline
if any OSD out of the total count in that CRUSH bucket is stopped. Also
standardize on capitalized acronyms (PG, OSD, CRUSH) and wrap the bucket
name in single quotes for better log readability.

Fixes: https://tracker.ceph.com/issues/74612
Signed-off-by: Sridhar Seshasayee <sridhar.seshasayee@ibm.com>