cmd/go: make go line declare minimum required Go version
For #57001, enforce the go line as declaring the minimum required
version of Go that can compile a module.
Modules that maintain compatibility with old versions of Go
but want to make use of new features in //go:build-constrained files
will be able to do so: the //go:build constraint will be interpreted
as changing the minimum Go version for that file and will unlock
the Go features allowed in that version.