remove 1.15 (sourcegraph#2270)

Author: davejrt

Diff for: tests/integration/fresh/fresh_test.go

@@ -21,7 +21,7 @@ func TestFreshDeployment(t *testing.T) {
 	noCleanup := os.Getenv("NOCLEANUP") == "true"
 
 	// Validate on various kubernetes versions
-	for _, k8sVersion := range []string{"1.15", "1.16", "1.17", "1.18"} {
+	for _, k8sVersion := range []string{"1.16", "1.17", "1.18"} {
 		k8sVersion := k8sVersion
 		t.Run(fmt.Sprintf("GKE version %q", k8sVersion), func(t *testing.T) {
 			config, err := commonConfig()