ci: drop macos/amd64 release builds

[barnabasbusa]

Summary

• Remove the build_darwin_amd64_binary job from the shared build workflow
• Drop the darwin_amd64 tarball steps, release-notes table row, and files entry from snapshot/release workflows
• Mac builds are now arm64-only

Test plan

• Trigger snapshot build and confirm artifacts contain only windows_amd64, linux_amd64, linux_arm64, darwin_arm64
• On the next tagged release, confirm artifacts and release-notes table match