Fix go-to-protobuf wrt gengo/v2

There's some very fishy-smelling logic in here, but this commit is
trying to be as focused as possible.

The *.pb.go diffs are the "name" encoded in the descriptor.  The
descriptor blobs can be decoded by this program (thanks StackOverflow!):

```
package main

import (
	"bytes"
	"compress/gzip"
	"encoding/json"
	"fmt"
	"os"

	"io/ioutil"

	proto "github.com/golang/protobuf/proto"
	dpb "github.com/golang/protobuf/protoc-gen-go/descriptor"
)

func main() {
	m := map[string][]byte{
		"before": blobv1,
		"after":  blobv2,
	}
	arg := os.Args[1]
	dump(m[arg])
}

func dump(bytes []byte) {
	fd, err := decodeFileDesc(bytes)
	if err != nil {
		panic(err)
	}
	b, err := json.MarshalIndent(fd, "", "  ")
	if err != nil {
		panic(err)
	}
	fmt.Println(string(b))
}

// decompress does gzip decompression.
func decompress(b []byte) ([]byte, error) {
	r, err := gzip.NewReader(bytes.NewReader(b))
	if err != nil {
		return nil, fmt.Errorf("bad gzipped descriptor: %v", err)
	}
	out, err := ioutil.ReadAll(r)
	if err != nil {
		return nil, fmt.Errorf("bad gzipped descriptor: %v", err)
	}
	return out, nil
}

func decodeFileDesc(enc []byte) (*dpb.FileDescriptorProto, error) {
	raw, err := decompress(enc)
	if err != nil {
		return nil, fmt.Errorf("failed to decompress enc: %v", err)
	}

	fd := new(dpb.FileDescriptorProto)
	if err := proto.Unmarshal(raw, fd); err != nil {
		return nil, fmt.Errorf("bad descriptor: %v", err)
	}
	return fd, nil
}

var blobv1 = []byte{
	// insert proto "before" blob here
}

var blobv2 = []byte{
	// insert proto "after" blob here
}
```

Running this with "before" and "after" args, and diffing the output
yields something like:

```diff
--- /tmp/a	2023-12-23 23:57:04.748090836 -0800
+++ /tmp/b	2023-12-23 23:57:11.000040973 -0800
@@ -1,5 +1,5 @@
 {
-  "name": "k8s.io/kubernetes/vendor/k8s.io/api/admission/v1/generated.proto",
+  "name": "k8s.io/api/admission/v1/generated.proto",
   "package": "k8s.io.api.admission.v1",
   "dependency": [
     "github.com/gogo/protobuf/gogoproto/gogo.proto",
```

Kubernetes-commit: b0a70dec4ab4cb9f972cf39a81ca5e5555417227