Clean some legacy files and move some build files (#23699)

* Clean the "tools" directory. The "tools" directory contains only two
files, move them.
* The "external_renderer.go" works like "cat" command to echo Stdin to
Stdout , to help testing.
* The `// gobuild: external_renderer` is incorrect, there should be no
space: `//gobuild: external_renderer`
* The `fmt.Print(os.Args[1])` is not a well-defined behavior, and it's
never used.
* The "watch.sh" is for "make watch", it's somewhat related to "build"
* After this PR, there is no "tools" directory, the project root
directory looks slightly simpler than before.
* Remove the legacy "contrib/autoboot.sh", there is no
"gogs_supervisord.sh"
* Remove the legacy "contrib/mysql.sql", it's never mentioned anywhere.
* Remove the legacy "contrib/pr/checkout.go", it has been broken for
long time, and it introduces unnecessary dependencies of the main code
base.

build/backport-locales.go

@@ -1,3 +1,6 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
 //go:build ignore
 
 package main