Go accepts removal of string(int) conversion, targeting 1.28

The language spec is finally losing string(int), with Rob Pike's proposal to remove the conversion accepted and targeted for Go 1.28. It's a small thing that has confused newcomers for years (the bootstrap-era shortcut that turns 65 into "A" instead of "65"), and its retirement pairs neatly with the other accepted proposals landing today: a v2 signers API for x/crypto/ssh and goroutine labels on tests to make timeout debugging less of a guessing game.