chore(crc32c): set up release scripts

[daniel-sanche]

This PR:

• restores the release scripts for crc32c that were removed as part of the monorepo migration
• adds extra validation to make sure sdist-only releases will not accidentally be published
• adds a release block on crc32c, until we add matching backend configs
• move the supported versions list into a centralized location for build scripts, instead of having each script encode it separately

[gemini-code-assist]

Code Review

This pull request updates the release and build configurations for the google-crc32c package across Linux, macOS, and Windows. It introduces a single source of truth for supported Python versions (python_versions.yaml), ensures the pure-Python fallback is disabled during Kokoro CI builds, adds twine check validation for built wheels, and updates test scripts to output JUnit XML logs. The reviewer feedback highlights opportunities to improve the robustness of the build scripts by replacing fragile version string slicing with more reliable parsing methods (like cut or for /f) and replacing hardcoded Python 3.12 references with dynamic py -3 calls.