[MM-48010] Fetch Threads since parameter should return threads with new replies. (#22228)

* Fix

* Test fix and condition is GtOrEq

store/sqlstore/thread_store.go

@@ -314,7 +314,11 @@ func (s *SqlThreadStore) GetThreadsForUser(userId, teamId string, opts model.Get
 	}
 
 	if opts.Since > 0 {
-		query = query.Where(sq.GtOrEq{"ThreadMemberships.LastUpdated": opts.Since})
+		query = query.
+			Where(sq.Or{
+				sq.GtOrEq{"ThreadMemberships.LastUpdated": opts.Since},
+				sq.GtOrEq{"Threads.LastReplyAt": opts.Since},
+			})
 	}
 
 	if opts.Unread {