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>

client/pull.go

@@ -197,7 +197,10 @@ func (c *Client) fetch(ctx context.Context, rCtx *RemoteContext, ref string, lim
 	)
 
 	if desc.MediaType == images.MediaTypeDockerSchema1Manifest && rCtx.ConvertSchema1 {
-		schema1Converter := schema1.NewConverter(store, fetcher)
+		schema1Converter, err := schema1.NewConverter(store, fetcher)
+		if err != nil {
+			return images.Image{}, fmt.Errorf("failed to get converter for %q: %w", ref, err)
+		}
 
 		handler = images.Handlers(append(rCtx.BaseHandlers, schema1Converter)...)