Implement FSFE REUSE for golang files (#21840)

Change all license headers to comply with REUSE specification.

Fix #16132

Co-authored-by: flynnnnnnnnnn <flynnnnnnnnnn@github>
Co-authored-by: John Olheiser <john.olheiser@gmail.com>

modules/structs/user_key.go

@@ -1,6 +1,5 @@
 // Copyright 2015 The Gogs Authors. All rights reserved.
-// Use of this source code is governed by a MIT-style
-// license that can be found in the LICENSE file.
+// SPDX-License-Identifier: MIT
 
 package structs