Relicense to GPL-3.0-only and add SPDX license headers (#87)

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 PDF Export | (c) 2019 Icinga GmbH | GPLv2 */
+// SPDX-FileCopyrightText: 2019 Icinga GmbH <https://icinga.com>
+// SPDX-License-Identifier: GPL-3.0-or-later
 
 namespace Icinga\Module\Pdfexport\Controllers;
 

LICENSE.md

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

application/controllers/ConfigController.php

@@ -1,6 +1,7 @@
 <?php
 
-/* Icinga PDF Export | (c) 2019 Icinga GmbH | GPLv2 */
+// SPDX-FileCopyrightText: 2019 Icinga GmbH <https://icinga.com>
+// SPDX-License-Identifier: GPL-3.0-or-later
 
 /** @var \Icinga\Application\Modules\Module $this */
 

application/forms/ChromeBinaryForm.php

@@ -1,6 +1,7 @@
 <?php
 
-/* Icinga PDF Export | (c) 2018 Icinga GmbH | GPLv2 */
+// SPDX-FileCopyrightText: 2018 Icinga GmbH <https://icinga.com>
+// SPDX-License-Identifier: GPL-3.0-or-later
 
 namespace Icinga\Module\Pdfexport;
 

configuration.php

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

library/Pdfexport/HeadlessChrome.php

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

library/Pdfexport/PrintStyleSheet.php

@@ -1,6 +1,7 @@
 <?php
 
-/* Icinga PDF Export | (c) 2018 Icinga GmbH | GPLv2 */
+// SPDX-FileCopyrightText: 2018 Icinga GmbH <https://icinga.com>
+// SPDX-License-Identifier: GPL-3.0-or-later
 
 namespace Icinga\Module\Pdfexport\ProvidedHook;
 

library/Pdfexport/PrintableHtmlDocument.php

@@ -1,6 +1,7 @@
 <?php
 
-/* Icinga PDF Export | (c) 2018 Icinga GmbH | GPLv2 */
+// SPDX-FileCopyrightText: 2018 Icinga GmbH <https://icinga.com>
+// SPDX-License-Identifier: GPL-3.0-or-later
 
 namespace Icinga\Module\Pdfexport;