Relicense to GPL-3.0-only and add SPDX license headers (#1338)

Relicense this module to `GPL-3.0-only`. Replace the old license file
with `LICENSE.md` and update documentation references accordingly.
Add `GPL-3.0-or-later` SPDX license headers to source files, allowing
relicensing under future GPL versions. This ensures compatibility with
third-party dependencies (e.g. `Apache-2.0`) incompatible with
`GPL-2.0-only`.

Author: lippserd

LICENSE

@@ -1,6 +1,7 @@
 <?php
 
-/* Icinga DB Web | (c) 2023 Icinga GmbH | GPLv2 */
+// SPDX-FileCopyrightText: 2023 Icinga GmbH <https://icinga.com>
+// SPDX-License-Identifier: GPL-3.0-or-later
 
 namespace Icinga\Module\Icingadb\Clicommands;
 

LICENSE.md

@@ -1,6 +1,7 @@
 <?php
 
-/* Icinga DB Web | (c) 2021 Icinga GmbH | GPLv2 */
+// SPDX-FileCopyrightText: 2021 Icinga GmbH <https://icinga.com>
+// SPDX-License-Identifier: GPL-3.0-or-later
 
 namespace Icinga\Module\Icingadb\Controllers;
 

README.md

@@ -1,6 +1,7 @@
 <?php
 
-/* Icinga DB Web | (c) 2020 Icinga GmbH | GPLv2 */
+// SPDX-FileCopyrightText: 2019 Icinga GmbH <https://icinga.com>
+// SPDX-License-Identifier: GPL-3.0-or-later
 
 namespace Icinga\Module\Icingadb\Controllers;
 

application/clicommands/MigrateCommand.php

@@ -1,6 +1,7 @@
 <?php
 
-/* Icinga DB Web | (c) 2020 Icinga GmbH | GPLv2 */
+// SPDX-FileCopyrightText: 2019 Icinga GmbH <https://icinga.com>
+// SPDX-License-Identifier: GPL-3.0-or-later
 
 namespace Icinga\Module\Icingadb\Controllers;
 

application/controllers/CommandTransportController.php

@@ -1,6 +1,7 @@
 <?php
 
-/* Icinga DB Web | (c) 2020 Icinga GmbH | GPLv2 */
+// SPDX-FileCopyrightText: 2019 Icinga GmbH <https://icinga.com>
+// SPDX-License-Identifier: GPL-3.0-or-later
 
 namespace Icinga\Module\Icingadb\Controllers;
 

application/controllers/CommentController.php

@@ -1,6 +1,7 @@
 <?php
 
-/* Icinga DB Web | (c) 2020 Icinga GmbH | GPLv2 */
+// SPDX-FileCopyrightText: 2019 Icinga GmbH <https://icinga.com>
+// SPDX-License-Identifier: GPL-3.0-or-later
 
 namespace Icinga\Module\Icingadb\Controllers;
 

application/controllers/CommentsController.php

@@ -1,6 +1,7 @@
 <?php
 
-/* Icinga DB Web | (c) 2020 Icinga GmbH | GPLv2 */
+// SPDX-FileCopyrightText: 2019 Icinga GmbH <https://icinga.com>
+// SPDX-License-Identifier: GPL-3.0-or-later
 
 namespace Icinga\Module\Icingadb\Controllers;