OpenAPIv2.go and OpenAPIv3.go both copy mutexes, generating racy code

It seems like the openapiv2 and openapiv3 codebases include generated code that violates the [Go copylock code analysis](https://pkg.go.dev/golang.org/x/tools/go/analysis/passes/copylock). (That code analysis checks to make sure that sync.Mutex is used with a pointer and isn't copied by value, rendering it unable to protect data.)

That code seems to be generated, so it might be a bug in the generator (I believe protoc-gen-go includes those sync.Mutex's explicitly to keep MessageState from being copied like this).

The errors look like:

```
openapiv2/OpenAPIv2.go:5872:10: assignment copies lock value to *m: github.com/google/gnostic-models/openapiv2.JsonReference contains google.golang.org/protobuf/internal/impl.MessageState contains sync.Mutex (copylocks)
openapiv2/OpenAPIv2.go:6248:11: assignment copies lock value to *m: github.com/google/gnostic-models/openapiv2.ParametersItem contains google.golang.org/protobuf/internal/impl.MessageState contains sync.Mutex (copylocks)
openapiv2/OpenAPIv2.go:6268:10: assignment copies lock value to *m: github.com/google/gnostic-models/openapiv2.PathItem contains google.golang.org/protobuf/internal/impl.MessageState contains sync.Mutex (copylocks)
openapiv2/OpenAPIv2.go:6542:11: assignment copies lock value to *m: github.com/google/gnostic-models/openapiv2.ResponseValue contains google.golang.org/protobuf/internal/impl.MessageState contains sync.Mutex (copylocks)
openapiv2/OpenAPIv2.go:6584:10: assignment copies lock value to *m: github.com/google/gnostic-models/openapiv2.Schema contains google.golang.org/protobuf/internal/impl.MessageState contains sync.Mutex (copylocks)
```

and

```
openapiv3/OpenAPIv3.go:6085:10: assignment copies lock value to *m: github.com/google/gnostic-models/openapiv3.PathItem contains google.golang.org/protobuf/internal/impl.MessageState contains sync.Mutex (copylocks)
openapiv3/OpenAPIv3.go:6213:10: assignment copies lock value to *m: github.com/google/gnostic-models/openapiv3.Reference contains google.golang.org/protobuf/internal/impl.MessageState contains sync.Mutex (copylocks)
```