all: update copyright and license headers

This updates all source files to use a new standard header for copyright
and license declaration.  Notably, copyright no longer includes a date,
and we now use the standard SPDX-License-Identifier header.

This commit was done almost entirely mechanically with perl, and then
some minimal manual fixes.

Updates #6865

Signed-off-by: Will Norris <will@tailscale.com>

cmd/tsconnect/src/app/index.css

@@ -1,6 +1,5 @@
-/* Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved. */
-/* Use of this source code is governed by a BSD-style */
-/* license that can be found in the LICENSE file. */
+/* Copyright (c) Tailscale Inc & AUTHORS */
+/* SPDX-License-Identifier: BSD-3-Clause */
 
 @import "xterm/css/xterm.css";