Specify license using SPDX qualifier

[sycloid]

This resolves gh-2159.

After this change 3 license-related warnings disappear.

• Have you provided a meaningful PR description?
• Have you added a test, reproducer or referred to an issue with a reproducer?
• Have you tested your changes locally for CPU and GPU devices?
• Have you made sure that new changes do not introduce compiler warnings?
• Have you checked performance impact of proposed changes?
• Have you added documentation for your changes, if necessary?
• Have you added your changes to the changelog?
• If this PR is a work in progress, are you opening the PR as a draft?

[sycloid]

With this change:

$ python scripts/build_locally.py --compiler-root=${HOME}/sycl-builds/test_llvm/build_sycl --c-compiler=${HOME}/sycl-builds/test_llvm/build_sycl/bin/clang --cxx-compiler=${HOME}/sycl-builds/test_llvm/build_sycl/bin/clang++
/home/sycloid/miniforge/envs/dev_dpctl/lib/python3.13/site-packages/setuptools/_distutils/cmd.py:90: DevelopDeprecationWarning: develop command is deprecated.
!!

        ********************************************************************************
        Please avoid running ``setup.py`` and ``develop``.
        Instead, use standards-based tools like pip or uv.

        By 2025-Oct-31, you need to update your project and remove deprecated calls
        or your builds will no longer be supported.

        See https://github.com/pypa/setuptools/issues/917 for details.
        ********************************************************************************

!!
  self.initialize_options()
[0/1] Re-running CMake...
-- dpcpp ver[0]: clang version 21.0.0git (https://github.com/intel/llvm 611e24571eab5d0bebeea859200484e68ba910ff)
-- dpcpp ver[1]: Target: x86_64-unknown-linux-gnu
<snipped>

[intel-python-devops]

Can one of the admins verify this patch?

[ndgrigorian]

LGTM

[coveralls]

coverage: 85.262%. remained the same
when pulling 28996ed on sycloid:resolve-license-deprecation-warnings
into 878cc19 on IntelPython:master.

[antonwolfy]

Thank you @sycloid