Consolidate crypto backend GOEXPERIMENTs into systemcrypto

[gdams]

Remove the per-platform GOEXPERIMENTs (opensslcrypto, cngcrypto, darwincrypto) and use only systemcrypto with platform build constraints.

Before this change, there was lots of confusion around the platform-specific experiements:

• Linux: GOEXPERIMENT=opensslcrypto
• Windows: GOEXPERIMENT=cngcrypto
• Darwin: GOEXPERIMENT=darwincrypto

Platform-specific code now uses build constraints like:
//go:build goexperiment.systemcrypto && linux
//go:build goexperiment.systemcrypto && windows
//go:build goexperiment.systemcrypto && darwin

Runtime checks use:
goexperiment.SystemCrypto && runtime.GOOS == "linux"

This simplifies the developer experience and makes cross-platform scripts and documentation easier to maintain.

Changes:

• Throws errors for OpenSSLCrypto, CNGCrypto, DarwinCrypto in goexperiment.Flags
• Update all build constraints to use systemcrypto + platform
• Update runtime checks to use SystemCrypto + runtime.GOOS
• Update test infrastructure to only check for systemcrypto

[qmuntal]

Docs need to be updated (including release notes, why not 🐱 )

[github-actions]

Patch Consistency Review ✅

I've completed a thorough review of the patch file changes in this PR. The changes consistently consolidate the platform-specific crypto backend GOEXPERIMENTs (opensslcrypto, cngcrypto, darwincrypto) into the unified systemcrypto experiment.

Summary of Changes

Patch 0002 (Add crypto backend GOEXPERIMENTs):

• Removes build tag logic that made systemcrypto automatically enable platform-specific experiments
• Removes test data for platform-specific experiments (e.g., backendtags_openssl)
• Keeps experiment definitions but simplifies the implementation

Patch 0003 (Implement crypto internal backend):

• Adds error checking to throw helpful errors when deprecated experiments are used
• Updates test infrastructure to only check for systemcrypto instead of platform-specific variants
• Simplifies iOS and test directory skip logic to use only systemcrypto

Patch 0004 (Use crypto backends):

• Changes build constraints from goexperiment.darwincrypto to goexperiment.systemcrypto && darwin
• Updates test skip conditions to use systemcrypto instead of platform-specific checks
• Simplifies static linking check on Linux

Consistency Check Results

✅ Vendor patches: No vendor-related changes detected
✅ Patch naming: All patches follow the established naming convention
✅ Content consistency: Changes are consistently applied across all three patches with no redundancy or missing updates

Conclusion

The patches maintain excellent cross-patch consistency. The consolidation is implemented uniformly across all affected areas, and the error handling for deprecated experiments provides a good migration path for users. No issues found.

AI generated by Patch Consistency Review Agent