pyproject: Use new format to specify licenses

See:
https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license

This eliminates the following warning messages when building the package:

********************************************************************************
Please use a simple string containing a SPDX expression for `project.license`.
You can also use `project.license-files`. (Both options available on
setuptools>=77.0.0).

By 2026-Feb-18, you need to update your project and remove deprecated calls
or your builds will no longer be supported.

See
https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license
for details.
********************************************************************************

Tests:

- Debian package can be built successfully. Two fewer warning during python
package build step were noticed.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

pyproject.toml

@@ -1,7 +1,8 @@
 [project]
 name = "plinth"
 description = "A web front end for administering FreedomBox"
-license = {file = "COPYING.md"}
+license = "AGPL-3.0-or-later"
+license-files = ["COPYING.md"]
 dynamic = ["version"]
 classifiers = [
     "Development Status :: 5 - Production/Stable",
@@ -9,8 +10,6 @@ classifiers = [
     "Environment :: Web Environment",
     "Framework :: Django",
     "Intended Audience :: End Users/Desktop",
-    "License :: DFSG approved",
-    "License :: OSI Approved :: GNU Affero General Public License v3 or later (AGPLv3+)",
     "Natural Language :: English",
     "Operating System :: POSIX :: Linux",
     "Programming Language :: JavaScript",