Update github.com/containerd/containerd

$ go get -u github.ccom/containerd/containerd
$ make vendor

... to silence warnings about https://github.com/advisories/GHSA-crp2-qrr5-8pq7 ,
in code we don't use.

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

vendor/google.golang.org/grpc/attributes/attributes.go

@@ -69,7 +69,9 @@ func (a *Attributes) Value(key interface{}) interface{} {
 // bool' is implemented for a value in the attributes, it is called to
 // determine if the value matches the one stored in the other attributes.  If
 // Equal is not implemented, standard equality is used to determine if the two
-// values are equal.
+// values are equal. Note that some types (e.g. maps) aren't comparable by
+// default, so they must be wrapped in a struct, or in an alias type, with Equal
+// defined.
 func (a *Attributes) Equal(o *Attributes) bool {
 	if a == nil && o == nil {
 		return true