Deprecate built-in aufs snapshotter
aufs was removed from Ubuntu kernel in Ubuntu 21.04, so deprecate the
built-in aufs snapshotter.
The aufs snapshotter can be still maintained as a non-builtin snapshotter.
@@ -326,3 +326,4 @@ The deprecated features are shown in the following table:
| Runtime V1 API and implementation (`io.containerd.runtime.v1.linux`) | containerd v1.4 | containerd v2.0 |
| Runc V1 implementation of Runtime V2 (`io.containerd.runc.v1`) | containerd v1.4 | containerd v2.0 |
| config.toml `version = 1` | containerd v1.5 | containerd v2.0 |
+| Built-in `aufs` snapshotter | containerd v1.5 | containerd v2.0 |
