MM-42535: Set ThreadAutoFollow to true by default (#19797)

Summary
A number of performance improvements shipped in v6.5 to support CRT with the ThreadAutoFollow bookkeeping enabled but the frontend features disabled. To prepare for a later enabling of the frontend features by default, we want to encourage customers to re-enable ThreadAutoFollow, and plan to enable this by default in v6.6.

Ticket Link
https://mattermost.atlassian.net/browse/MM-42535

model/config.go

@@ -781,7 +781,7 @@ func (s *ServiceSettings) SetDefaults(isUpdate bool) {
 	}
 
 	if s.ThreadAutoFollow == nil {
-		s.ThreadAutoFollow = NewBool(false)
+		s.ThreadAutoFollow = NewBool(true)
 	}
 
 	if s.CollapsedThreads == nil {