@google-cloud/aiplatform using very old protobuf.js library causing issues with gRPC

Hello there.

I am attempting to run 'createCustomJob' in the '@google-cloud/aiplatform' platform package, and I receive the following error when passing a numeric or string value of 'replicaCount' as part of a 'workerPoolSpec':

```
Error: 13 INTERNAL: Request message serialization failure: util.Long.fromValue is not a function
```

On further investigation, it appears that the '@google-cloud/aiplatform' library is using 'protobuf.js' (a library that hasn't been updated in ten years), instead of 'protobufjs' which is much more modern and used by libraries such as 'google-gax'.

'Protobuf.js' itself uses a version of 'long' that is also out of date, which is probably responsible for the error above.

Ref: https://github.com/googleapis/google-cloud-node/blame/823df1eca530906515ee7f631054e845af7108c5/packages/google-cloud-aiplatform/package.json#L57

Would be great if the code could be updated to use the 'protobufjs' library instead of 'protobuf.js'. Thanks!