fix: use spdx license classifier

[IamPhytan]

When running building the sources with the developer instructions, I receive this error message:

--- stderr: colcon-python-setup-py                                                                          
/home/damien/code/ros/colcon/.venv/lib/python3.14/site-packages/setuptools/dist.py:759: SetuptoolsDep
recationWarning: 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()
---

Changed the license specifier, like in colcon-output.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.00%. Comparing base (45d0a04) to head (8f657a1).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #67   +/-   ##
=======================================
  Coverage   51.00%   51.00%           
=======================================
  Files           3        3           
  Lines         149      149           
  Branches       23       23           
=======================================
  Hits           76       76           
  Misses         67       67           
  Partials        6        6           

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