Officially deprecate Schema 1

Schema 1 has been substantially deprecated since circa. 2017 in favor of Schema 2 introduced in Docker 1.10 (Feb 2016)
and its successor OCI Image Spec v1, but we have not officially deprecated Schema 1.

One of the reasons was that Quay did not support Schema 2 so far, but it is reported that Quay has been
supporting Schema 2 since Feb 2020 (moby/buildkit issue 409).

This PR deprecates pulling Schema 1 images but the feature will not be removed before containerd 2.0.
Pushing Schema 1 images was never implemented in containerd (and its consumers such as BuildKit).

Docker/Moby already disabled pushing Schema 1 images in Docker 20.10 (moby/moby PR 41295),
but Docker/Moby has not yet disabled pulling Schema 1 as containerd has not yet deprecated Schema 1.
(See the comments in moby/moby PR 42300.)
Docker/Moby is expected to disable pulling Schema 1 images in future after this deprecation.

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

cmd/ctr/commands/content/fetch.go

@@ -190,7 +190,7 @@ func Fetch(ctx context.Context, client *containerd.Client, ref string, config *F
 		containerd.WithPullLabels(labels),
 		containerd.WithResolver(config.Resolver),
 		containerd.WithImageHandler(h),
-		containerd.WithSchema1Conversion,
+		containerd.WithSchema1Conversion, //nolint:staticcheck // Ignore SA1019. Need to keep deprecated package for compatibility.
 	}
 	opts = append(opts, config.RemoteOpts...)