MM-31715: Flip ExperimentalGossip to be true by default (#16644)

* MM-31715: Flip ExperimentalGossip to be true by default

https://mattermost.atlassian.net/browse/MM-31715

```release-notes
The UseExperimentalGossip field under ClusterSettings is now true by default.
This means that new installations will use the gossip protocol for cluster communication.
There will be no changes to existing installations.
```

* update test config

model/config.go

@@ -848,7 +848,7 @@ func (s *ClusterSettings) SetDefaults() {
 	}
 
 	if s.UseExperimentalGossip == nil {
-		s.UseExperimentalGossip = NewBool(false)
+		s.UseExperimentalGossip = NewBool(true)
 	}
 
 	if s.EnableExperimentalGossipEncryption == nil {

tests/test-config.json

@@ -320,7 +320,7 @@
         "ClusterName": "",
         "OverrideHostname": "",
         "UseIpAddress": true,
-        "UseExperimentalGossip": false,
+        "UseExperimentalGossip": true,
         "ReadOnlyConfig": true,
         "GossipPort": 8074,
         "StreamingPort": 8075,