[MM-41351] Set ThreadAutoFollow to false by default (#19441)

Summary
ThreadAutoFollow is now false by default. Won't affect existing systems that have it set to true.

Ticket Link
https://mattermost.atlassian.net/browse/MM-41351

model/config.go

@@ -772,7 +772,7 @@ func (s *ServiceSettings) SetDefaults(isUpdate bool) {
 	}
 
 	if s.ThreadAutoFollow == nil {
-		s.ThreadAutoFollow = NewBool(true)
+		s.ThreadAutoFollow = NewBool(false)
 	}
 
 	if s.CollapsedThreads == nil {