Change the license field in pyproject.toml to SPDX string.

According to the deprecation warning, support for the old way of declaring the license will be removed from setuptools on 2026-Feb-18.

PiperOrigin-RevId: 791716539
Change-Id: Iaf9ec55f9425380b017bd7ea370cd481ed01aa8f

python/pyproject.toml

@@ -10,12 +10,11 @@ authors = [
 ]
 description = "MuJoCo Physics Simulator"
 requires-python = ">=3.9"
-license = {text = "Apache License 2.0"}
+license = "Apache-2.0"
 classifiers = [
     "Development Status :: 5 - Production/Stable",
     "Intended Audience :: Developers",
     "Intended Audience :: Science/Research",
-    "License :: OSI Approved :: Apache Software License",
     "Natural Language :: English",
     "Programming Language :: Python :: 3",
     "Programming Language :: Python :: 3.9",