Update github.com/containerd/containerd to v1.5.7

... to include a fix for
https://github.com/advisories/GHSA-c2h3-6mxw-7mvq .

(Note that Skopeo doesn't depend on the vulnerable code,
so this is primarily to avoid dependency checker warnings.)

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

vendor/google.golang.org/grpc/internal/channelz/funcs.go

@@ -630,7 +630,7 @@ func (c *channelMap) GetServerSockets(id int64, startID int64, maxResults int64)
 	if count == 0 {
 		end = true
 	}
-	var s []*SocketMetric
+	s := make([]*SocketMetric, 0, len(sks))
 	for _, ns := range sks {
 		sm := &SocketMetric{}
 		sm.SocketData = ns.s.ChannelzMetric()