Do not use weak import

According to https://github.com/protocolbuffers/protobuf/issues/9184

> Weak fields are an old and deprecated internal-only feature that we never
> open sourced.

This blocks us to upgrade protoc.

Fixes #6232.

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

api/types/mount.proto

@@ -18,7 +18,7 @@ syntax = "proto3";
 
 package containerd.types;
 
-import weak "gogoproto/gogo.proto";
+import "gogoproto/gogo.proto";
 
 option go_package = "github.com/containerd/containerd/api/types;types";