Use PEP 639 license information for Ruff itself instead of classifier (#19499)

## Summary

Declare licenses using only these two fields, as per PEP 639:
* `license`: SPDX license expression consisting of one or more license
identifiers
* `license-files`: list of license file glob patterns

Supported by maturin ≥ 1.9.0:
https://www.maturin.rs/changelog.html

## Test Plan

N/A

pyproject.toml

@@ -1,5 +1,5 @@
 [build-system]
-requires = ["maturin>=1.0,<2.0"]
+requires = ["maturin>=1.9,<2.0"]
 build-backend = "maturin"
 
 [project]
@@ -9,7 +9,8 @@ description = "An extremely fast Python linter and code formatter, written in Ru
 authors = [{ name = "Astral Software Inc.", email = "hey@astral.sh" }]
 readme = "README.md"
 requires-python = ">=3.7"
-license = { file = "LICENSE" }
+license = "MIT"
+license-files = ["LICENSE"]
 keywords = [
     "automation",
     "flake8",
@@ -22,7 +23,6 @@ classifiers = [
     "Development Status :: 5 - Production/Stable",
     "Environment :: Console",
     "Intended Audience :: Developers",
-    "License :: OSI Approved :: MIT License",
     "Operating System :: OS Independent",
     "Programming Language :: Python",
     "Programming Language :: Python :: 3.7",