vendor: update oci image spec dependency

Signed-off-by: Stephen J Day <stephen.day@docker.com>

vendor/github.com/opencontainers/image-spec/specs-go/versioned.go

@@ -20,7 +20,4 @@ package specs
 type Versioned struct {
 	// SchemaVersion is the image manifest schema that this image follows
 	SchemaVersion int `json:"schemaVersion"`
-
-	// MediaType is the media type of this schema.
-	MediaType string `json:"mediaType"`
 }