spec: update to latest version before releasing Go 1.21

[griesemer]

Reminder issue to (wholesale) copy the spec ($GOROOT/doc/go_spec.html) from tip into the release branch so the released spec is up-to-date. Marking as release blocker so we don't forget.

This needs to be done after CL 513275 has been submitted to tip.

(This is easier than finding the two or three cherry-picks, also no potential 1.22 changes have made it into the spec.)

[gopherbot]

Change https://go.dev/cl/514435 mentions this issue: [release-branch.go1.21] spec: update spec to version at tip

[gopherbot]

Closed by merging 4a14d9c to release-branch.go1.21.