fix: use spdx license classifier

[IamPhytan]

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

--- stderr: colcon-library-path                                                                          
/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 100.00%. Comparing base (84f8fc2) to head (a6222d6).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #27   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           26        26           
  Branches         5         5           
=========================================
  Hits            26        26           

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