cmd/go: error out of 'go mod tidy' if the go.mod file specifies a newer-than-supported Go version [1.16 backport] #46144
Description
@bcmills requested issue #46142 to be considered for backport to the next 1.16 minor release.
@gopherbot, please backport to Go 1.15 and 1.16. We don't want users on older Go releases to accidentally break the invariants of the modules they're working in.