respond to warning: License classifiers are deprecated

[vchudnov-g]

When locally running nox -r -s mypy-3.13, I see a warning about license classifiers. We should figure out what the best practice is now and adapt.

The relevant part of the output containing the warning was:

Building wheels for collected packages: gapic-generator
  Building wheel for gapic-generator (pyproject.toml): started
  Building wheel for gapic-generator (pyproject.toml): finished with status 'error'
  error: subprocess-exited-with-error
  
  × Building wheel for gapic-generator (pyproject.toml) did not run successfully.
  │ exit code: 1
  ╰─> [45 lines of output]
      /tmp/pip-build-env-w8s9swaq/overlay/lib/python3.13/site-packages/setuptools/dist.py:759: SetuptoolsDeprecationWarning: License classifiers are deprecated.
      !!
      
              ********************************************************************************
              Please consider removing the following classifiers in favor of a SPDX license expression:
      
              License :: OSI Approved :: Apache Software License
      
              See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
              ********************************************************************************
      
      !!
        self._finalize_license_expression()