pyproject.toml: remove deprecated License classifier for PEP 639

[jsmolic]

This is needed to be able to build the project using setuptools >77. Without it, conda package fails to build with:

 │ │     File "$PREFIX/lib/python3.11/site-packages/setuptools/config/_apply_pyprojecttoml.py", line 61, in apply
 │ │       dist._finalize_license_expression()
 │ │     File "$PREFIX/lib/python3.11/site-packages/setuptools/dist.py", line 430, in _finalize_license_expression
 │ │       raise InvalidConfigError(
 │ │   setuptools.errors.InvalidConfigError: License classifiers have been superseded by license expressions (see https://peps.python.o
 │ │ rg/pep-0639/). Please remove:
 │ │   License :: OSI Approved :: Apache Software License
 │ │   error: subprocess-exited-with-error
 │ │
 │ │   × Preparing metadata (pyproject.toml) did not run successfully.
 │ │   │ exit code: 1
 │ │   Preparing metadata (pyproject.toml): finished with status 'error'
 │ │   ╰─> No available output.