[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

api4/user_test.go

@@ -5947,6 +5947,11 @@ func TestFollowThreads(t *testing.T) {
 	th := Setup(t).InitBasic()
 	defer th.TearDown()
 
+	th.App.UpdateConfig(func(cfg *model.Config) {
+		*cfg.ServiceSettings.ThreadAutoFollow = true
+		*cfg.ServiceSettings.CollapsedThreads = model.CollapsedThreadsDefaultOn
+	})
+
 	t.Run("1 thread", func(t *testing.T) {
 		client := th.Client