cmd/go: go get go@1.21 should work better

[rsc]

In a go 1.12 module using Go built at tip:

% go get go@1.21
go: go@1.21: no matching versions for query "1.21"

It seems like since the local Go toolchain knows Go 1.21, this should be permitted.

[gopherbot]

Change https://go.dev/cl/509097 mentions this issue: cmd/go: fix go get go@badversion

[gopherbot]

Change https://go.dev/cl/509217 mentions this issue: cmd/go/internal/modfetch: always allow Stat for the current toolchain to succeed