go back to using the "or any later version" clause of GPLv3

I think arguments that "Such new versions will be similar in spirit to
the present version", in my own reading and readings such as in
https://www.draketo.de/software/gpl-or-later, convince me for now that
it is acceptable to allow the "or later" for compatibility and future
problem's sake

Signed-off-by: Brian S. Stephan <bss@incorporeal.org>

incorporealcms/static/css/base.css

@@ -1,6 +1,6 @@
 /*
  * SPDX-FileCopyrightText: © 2020 Brian S. Stephan <bss@incorporeal.org>
- * SPDX-License-Identifier: GPL-3.0-only
+ * SPDX-License-Identifier: GPL-3.0-or-later
  */
 
 html {

incorporealcms/static/css/dark.css

@@ -1,6 +1,6 @@
 /*
  * SPDX-FileCopyrightText: © 2020 Brian S. Stephan <bss@incorporeal.org>
- * SPDX-License-Identifier: GPL-3.0-only
+ * SPDX-License-Identifier: GPL-3.0-or-later
  */
 
 /* common styling via the base.css, used in light and dark */

incorporealcms/static/css/light.css

@@ -1,6 +1,6 @@
 /*
  * SPDX-FileCopyrightText: © 2020 Brian S. Stephan <bss@incorporeal.org>
- * SPDX-License-Identifier: GPL-3.0-only
+ * SPDX-License-Identifier: GPL-3.0-or-later
  */
 
 /* common styling via the base.css, used in light and dark */

incorporealcms/static/css/plain.css

@@ -1,6 +1,6 @@
 /*
  * SPDX-FileCopyrightText: © 2020 Brian S. Stephan <bss@incorporeal.org>
- * SPDX-License-Identifier: GPL-3.0-only
+ * SPDX-License-Identifier: GPL-3.0-or-later
  */
 
 /* specify almost no styling, just fix some image and nav rendering */

incorporealcms/static/js/style_switcher.js

@@ -5,7 +5,7 @@
  * of my CMS, so I want to keep it around even in the static site.
  *
  * SPDX-FileCopyrightText: © 2025 Brian S. Stephan <bss@incorporeal.org>
- * SPDX-License-Identifier: GPL-3.0-only
+ * SPDX-License-Identifier: GPL-3.0-or-later
  */
 
 /**