Use SPDX license identifier in package metadata

[chala2001]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Sets the license metadata to the SPDX identifier Apache-2.0 instead of the free
text Apache License Version 2.0, and drops the deprecated
License :: OSI Approved :: Apache Software License classifier, per PEP 639.

The diff in the issue covers setup.py. setup-asyncio.py and setup-release.py
carried the same two values, so I changed those too - otherwise the asyncio and
release builds would keep emitting the old metadata.

Checked the generated metadata with setuptools 84.0.0, before and after:

before:  License: Apache License Version 2.0
         Classifier: License :: OSI Approved :: Apache Software License

after:   License: Apache-2.0

One thing worth flagging: setup(license="Apache-2.0") still lands in the legacy
License field rather than PEP 639's License-Expression. The value is a valid
SPDX identifier now and the deprecated classifier is gone, which is what the issue
asks for, but a real License-Expression would need a pyproject.toml with
[project] license. Happy to do that as a follow-up if you want it.

Which issue(s) this PR fixes:

Fixes #2686

Special notes for your reviewer:

Packaging metadata only, no code or behaviour changes.

Does this PR introduce a user-facing change?

Package license metadata now uses the SPDX identifier `Apache-2.0`, and the deprecated Apache Software License classifier has been removed.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: chala2001 / name: chala2001 (c2dd7d1)

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[yliaog]

thanks for the PR
please follow up on pyproject.toml with [project] license

/lgtm
/approve

[kubernetes-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chala2001, yliaog

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