Replace slices.Contains usage with helper func

The 8.12 branch is not yet on Go 1.21 so the slices package isn't available.

dev-tools/packaging/package_test.go

@@ -32,7 +32,6 @@ import (
 	"os"
 	"path/filepath"
 	"regexp"
-	"slices"
 	"strings"
 	"testing"
 
@@ -694,7 +693,7 @@ func readDocker(dockerFile string) (*packageFile, *dockerInfo, error) {
 			if err != nil {
 				return nil, nil, err
 			}
-		case slices.Contains(manifest.Layers, header.Name):
+		case sliceContains(manifest.Layers, header.Name):
 			layer, err := readTarContents(header.Name, tarReader)
 			if err != nil {
 				return nil, nil, err
@@ -828,3 +827,12 @@ func readDockerInfo(r io.Reader) (*dockerInfo, error) {
 
 	return &info, nil
 }
+
+func sliceContains(s []string, e string) bool {
+	for _, v := range s {
+		if e == v {
+			return true
+		}
+	}
+	return false
+}