They did something sneaky. In go 1.21, they changed the version number format in the `go.mod` files. So trying to build with go 1.18 will result in:

  go: errors parsing go.mod:
  go.mod:3: invalid go version '1.21.0': must match format 1.23

However, this will only happen if you use the `go mod init` to create your module. If you manually specify `go 1.21` in your `go.mod`, it will build without complaining.