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/protobuf/encoding/prototext/decode.go

@@ -744,9 +744,6 @@ func (d decoder) skipValue() error {
 				// Skip items. This will not validate whether skipped values are
 				// of the same type or not, same behavior as C++
 				// TextFormat::Parser::AllowUnknownField(true) version 3.8.0.
-				if err := d.skipValue(); err != nil {
-					return err
-				}
 			}
 		}
 	}