Use SPDX license identifiers.

This patch switches code to SPDX one-line license identifiers according to
https://spdx.dev/learn/handling-license-info/
and replacing long license text headers.

I used C++ format on the first line in style
// SPDX-License-Identifier: <id>
except exported libcryptsetup.h, when only C comments are used.

The only additional changes are:
- switch backend utf8.c from LGPL2+ to LGPL2.1+ (as in systemd)
- add some additional formatting lines.

lib/luks2/luks2_reencrypt_digest.c

@@ -1,23 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * LUKS - Linux Unified Key Setup v2, reencryption digest helpers
  *
  * Copyright (C) 2022-2024 Red Hat, Inc. All rights reserved.
  * Copyright (C) 2022-2024 Ondrej Kozina
  * Copyright (C) 2022-2024 Milan Broz
- *
- * This program is free software; you can redistribute it and/or
- * modify it under the terms of the GNU General Public License
- * as published by the Free Software Foundation; either version 2
- * of the License, or (at your option) any later version.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License
- * along with this program; if not, write to the Free Software
- * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
 #include "luks2_internal.h"