Disable the support for Schema 1 images

Schema 1 (`application/vnd.docker.distribution.manifest.v1+prettyjws`) has been
officially deprecated since containerd v1.7 (PR 6884).

We have planned to remove the support for Schema 1 in containerd v2.0, but this
removal may still surprise some users.
So, in containerd v2.0 we will just disable it by default.

The support for Schema 1 can be still enabled by setting an environment variable
`CONTAINERD_ENABLE_DEPRECATED_PULL_SCHEMA_1_IMAGE=1`, however, this workaround
will be completely removed in containerd v2.1.

Schema 2 was introduced in Docker 1.10 (Feb 2016), so most users should
have been already using Schema 2 or OCI.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

integration/client/client_unix_test.go

@@ -23,6 +23,7 @@ import (
 
 	. "github.com/containerd/containerd/v2/client"
 	"github.com/containerd/containerd/v2/integration/images"
+	"github.com/containerd/containerd/v2/pkg/deprecation"
 	"github.com/containerd/platforms"
 )
 
@@ -46,6 +47,7 @@ var (
 )
 
 func TestImagePullSchema1WithEmptyLayers(t *testing.T) {
+	t.Setenv(deprecation.EnvPullSchema1Image, "1")
 	client, err := newClient(t, address)
 	if err != nil {
 		t.Fatal(err)