ci: remove UV_PRERELEASE workaround once Python 3.15 is officially released #18532

Description

UV_PRERELEASE: allow is currently scoped to Python 3.15 in .github/workflows/unittest.yml to allow test runs on unreleased Python 3.15 to resolve pre-release dependencies and development wheels (such as grpcio, numpy, etc.).

Action Items

Once Python 3.15 is officially released GA and upstream binary wheels are published on PyPI:

• Remove the UV_PRERELEASE workaround for Python 3.15 in .github/workflows/unittest.yml.
• Ensure all Python matrix versions run under standard resolution without pre-releases allowed.