cmd/go: add go get go@version and toolchain@version
go get go@version and toolchain@version updates the
go and toolchain lines in go.mod. If toolchain ends up <= go,
it is dropped.
When the go version crosses certain version boundaries,
it may be necessary to run 'go mod tidy -go=version'.
That's left for a followup CL.
When the go or toolchain version ends up higher than the
current toolchain version, we cannot be sure we know how
to write the file out, so we fail with an error message.
In GOTOOLCHAIN auto mode, the newer toolchain should
be downloaded and reinvoked; that's left for a followup CL too.