PLT-6478 Fixed copyright date on login screen (#6329)

cmd/platform/config.go

@@ -1,4 +1,4 @@
-// Copyright (c) 2016 Mattermost, Inc. All Rights Reserved.
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
 // See License.txt for license information.
 package main