Refer to registry.k8s.io instead of k8s.gcr.io

... per https://kubernetes.io/blog/2023/02/06/k8s-gcr-io-freeze-announcement/ .

We are seeing intermittent failures (sufficient to reliably cause a test suite failure)
pulling from k8s.gcr.io, let's see if using the newer one improves things.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

integration/sync_test.go

@@ -25,15 +25,15 @@ const (
 	// A repository with a path with multiple components in it which
 	// contains multiple tags, preferably with some tags pointing to
 	// manifest lists, and with some tags that don't.
-	pullableRepo = "k8s.gcr.io/coredns/coredns"
+	pullableRepo = "registry.k8s.io/coredns/coredns"
 	// A tagged image in the repository that we can inspect and copy.
-	pullableTaggedImage = "k8s.gcr.io/coredns/coredns:v1.6.6"
+	pullableTaggedImage = "registry.k8s.io/coredns/coredns:v1.6.6"
 	// A tagged manifest list in the repository that we can inspect and copy.
-	pullableTaggedManifestList = "k8s.gcr.io/coredns/coredns:v1.8.0"
+	pullableTaggedManifestList = "registry.k8s.io/coredns/coredns:v1.8.0"
 	// A repository containing multiple tags, some of which are for
 	// manifest lists, and which includes a "latest" tag.  We specify the
 	// name here without a tag.
-	pullableRepoWithLatestTag = "k8s.gcr.io/pause"
+	pullableRepoWithLatestTag = "registry.k8s.io/pause"
 )
 
 func TestSync(t *testing.T) {
@@ -323,7 +323,7 @@ func (s *syncSuite) TestYamlRegex2Dir() {
 	dir1 := path.Join(tmpDir, "dir1")
 
 	yamlConfig := `
-k8s.gcr.io:
+registry.k8s.io:
   images-by-tag-regex:
     pause: ^[12]\.0$  # regex string test
 `
@@ -344,7 +344,7 @@ func (s *syncSuite) TestYamlDigest2Dir() {
 	dir1 := path.Join(tmpDir, "dir1")
 
 	yamlConfig := `
-k8s.gcr.io:
+registry.k8s.io:
   images:
     pause:
     - sha256:59eec8837a4d942cc19a52b8c09ea75121acc38114a2c68b98983ce9356b8610
@@ -362,7 +362,7 @@ func (s *syncSuite) TestYaml2Dir() {
 	dir1 := path.Join(tmpDir, "dir1")
 
 	yamlConfig := `
-k8s.gcr.io:
+registry.k8s.io:
   images:
     coredns/coredns:
       - v1.8.0