Consistent license message for all the go files (#13235)

* Consistent license message for all the go files

* Fixing the last set of unconsistencies with the license headers

* Addressing PR review comments

* Fixing busy.go and busy_test.go license header

services/httpservice/httpservice.go

@@ -1,5 +1,5 @@
 // Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
-// See License.txt for license information.
+// See LICENSE.txt for license information.
 
 package httpservice