Use Khepri by default in CT tests

When starting a new cluster, Khepri is the default metadata store in 4.2.
Let's also make Khepri the default metadata store in CT tests.

(cherry picked from commit 96d31ec)

Author: ansd

deps/rabbitmq_ct_helpers/src/rabbit_ct_broker_helpers.erl

@@ -1104,8 +1104,8 @@ configured_metadata_store(Config) ->
             mnesia;
         _ ->
             case os:getenv("RABBITMQ_METADATA_STORE") of
-                "khepri" -> khepri;
-                _        -> mnesia
+                "mnesia" -> mnesia;
+                _ -> khepri
             end
     end.