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/ssg.py

@@ -1,7 +1,7 @@
 """Build an instance as a static site suitable for serving via e.g. Nginx.
 
 SPDX-FileCopyrightText: © 2025 Brian S. Stephan <bss@incorporeal.org>
-SPDX-License-Identifier: GPL-3.0-only
+SPDX-License-Identifier: GPL-3.0-or-later
 """
 import argparse
 import logging