QQ: fix the key_metrics_rpc function.

Currently this function always falls back to the compatibility code
and never gets the benefit of using ra:key_metrics/1 due to incorrect
use of the map update operatior ":=" instead of the insert operator
"=>".

(cherry picked from commit 3b1ef8f)

Author: kjnilsson

deps/rabbit/src/rabbit_quorum_queue.erl

@@ -1160,7 +1160,7 @@ cluster_state(Name) ->
 
 key_metrics_rpc(ServerId) ->
     Metrics = ra:key_metrics(ServerId),
-    Metrics#{machine_version := rabbit_fifo:version()}.
+    Metrics#{machine_version => rabbit_fifo:version()}.
 
 -spec status(rabbit_types:vhost(), Name :: rabbit_misc:resource_name()) ->
     [[{binary(), term()}]] | {error, term()}.