Enable immutable releases on GitHub

[stefanb]

Starting with v3.12.0 you proudly enabled immutable images.
For the same reason of supply chain security it would make sense to enable release immutability on gtihub.

See:

• Docs https://docs.github.com/en/code-security/concepts/supply-chain-security/immutable-releases
• Blog post https://github.blog/changelog/2025-10-28-immutable-releases-are-now-generally-available/
• Example of downstream issue because a release tag was moved: nova-fairwinds: update checksum for 3.12.0 release Homebrew/homebrew-core#289178

Thank you!