Use proto.Marshal instead of calling Marshal() on objects

With google.golang.org/protobuf, proto-generated objects only have
ProtoReflect(). They don't have Marshal() anymore (see #6564).

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

cmd/containerd-shim/main_unix.go

@@ -39,6 +39,7 @@ import (
 	"github.com/containerd/containerd/namespaces"
 	"github.com/containerd/containerd/pkg/process"
 	"github.com/containerd/containerd/protobuf"
+	"github.com/containerd/containerd/protobuf/proto"
 	ptypes "github.com/containerd/containerd/protobuf/types"
 	shimlog "github.com/containerd/containerd/runtime/v1"
 	"github.com/containerd/containerd/runtime/v1/shim"
@@ -305,7 +306,7 @@ func (l *remoteEventsPublisher) Publish(ctx context.Context, topic string, event
 	if err != nil {
 		return err
 	}
-	data, err := encoded.Marshal()
+	data, err := proto.Marshal(encoded)
 	if err != nil {
 		return err
 	}