Don't use go.mod to detect go version

The declaration in go.mod is intended indicate the minimum compatible version:
https://go.dev/doc/modules/gomod-ref#go

The current logic is treating like an exact version specifier

PiperOrigin-RevId: 423183372
Change-Id: Ia522cdf1a5c89bedce6976b59a5e811c386f6c4c

Author: matthewrobertson

builders/gcp/base/acceptance/go_test.go

@@ -40,6 +40,7 @@ func TestAcceptanceGo(t *testing.T) {
 			App:        "go/simple_gomod",
 			MustUse:    []string{goRuntime, goBuild},
 			MustNotUse: []string{goPath},
+			MustOutput: []string{"Using latest runtime version:"},
 		},
 		{
 			Name:       "Go.mod package",

builders/testdata/generic/go/simple_gomod/go.mod

@@ -1,2 +1,3 @@
 module example.com/package
 
+go 1.14

cmd/go/runtime/BUILD.bazel

@@ -26,7 +26,6 @@ go_binary(
     deps = [
         "//pkg/env",
         "//pkg/gcpbuildpack",
-        "//pkg/golang",
         "//pkg/runtime",
         "@com_github_buildpacks_libcnb//:go_default_library",
     ],

cmd/go/runtime/main.go

@@ -25,7 +25,6 @@ import (
 
 	"github.com/GoogleCloudPlatform/buildpacks/pkg/env"
 	gcp "github.com/GoogleCloudPlatform/buildpacks/pkg/gcpbuildpack"
-	"github.com/GoogleCloudPlatform/buildpacks/pkg/golang"
 	"github.com/GoogleCloudPlatform/buildpacks/pkg/runtime"
 	"github.com/buildpacks/libcnb"
 )
@@ -95,10 +94,6 @@ func runtimeVersion(ctx *gcp.Context) (string, error) {
 		ctx.Logf("Using runtime version from %s: %s", env.RuntimeVersion, version)
 		return version, nil
 	}
-	if version := golang.GoModVersion(ctx); version != "" {
-		ctx.Logf("Using runtime version from go.mod: %s", version)
-		return version, nil
-	}
 	version, err := latestGoVersion(ctx)
 	if err != nil {
 		return "", fmt.Errorf("getting latest version: %w", err)