Mutation responses should be required

### Version Information

Server Version: v2.16.1

### Environment

Cloud and OSS

### What is the current behaviour?

Generated mutations have non-required output types.

For example:

### What is the expected behaviour?

I would expect instead to have:
```graphql
insert_users(objects: [users_insert_input!]!on_conflict: users_on_conflict): users_mutation_response!
```

Indeed, [the documentation ](https://hasura.io/docs/latest/api-reference/graphql-api/mutation/#insert-upsert-syntax) specifies that mutation responses are required:

This might not seem important but when doing Typescript codegen, we end up with nullable response types which is a bit annoying.

### How to reproduce the issue?
1. check the Docs explorer of Hasura with any generated mutations

### Keywords

mutation response output types required non-nullable

Start in the Hasura Docs explorer with a generated mutation and compare its output type with the documented insert/upsert syntax. Trace the schema generation path that defines mutation response nullability, then verify that generated mutation responses are non-nullable and that TypeScript codegen no longer produces nullable response types.

Written by the indexing model from the issue text.