Use PEP 639 license metadata for uv itself (#16477)

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## Summary

In [pep 639](https://peps.python.org/pep-0639/) the classifier for
licenses gets deprecated in favor of the license field.
The benefits for UV:
- Clear that you can pick a license, not have to have both
- Clear it is version 2 for the Apache license

https://peps.python.org/pep-0639/

## Test Plan

Builds, works.
If the license shows up in pypi.org with the SPDX operator, it works

pyproject.toml

@@ -11,13 +11,13 @@ requires-python = ">=3.8"
 keywords = [
   "uv", "requirements", "packaging"
 ]
+license="MIT OR Apache-2.0"
+license-files = ["LICENSE-APACHE", "LICENSE-MIT"]
 classifiers = [
   "Development Status :: 5 - Production/Stable",
   "Environment :: Console",
   "Intended Audience :: Developers",
   "Operating System :: OS Independent",
-  "License :: OSI Approved :: MIT License",
-  "License :: OSI Approved :: Apache Software License",
   "Programming Language :: Python",
   "Programming Language :: Python :: 3.8",
   "Programming Language :: Python :: 3.9",