lang/go: Add Go 1.21 port from upstream and bump default version

[RoundofThree]

• Cherry-pick go121 from upstream
• Apply the same patches as in #156
• Bump the default go version to 1.21 (matching upstream)

[brooksdavis]

Please redo the cherry-picks with the -x option so the upstream commit is recorded.

[kwitaszczyk]

I won't backport it to 24.05 but the next package repository for dev will include this change.