@policy directive is not recognized while loading the schema to generate code

### What happened?
Created the schema with the new federation spec.

```
extend schema
@link(
url: "https://specs.apollo.dev/federation/v2.6",
import: [
"@key",
"@policy"
]
)

...
...
type Customer {
...
last4ssn: String! @policy(policies: "testPolicy")
...
}
...
...
```

When I executed the generate command it failed with the below error.
```
failed to load schema: graph/customer.graphqls:21: Undefined directive policy.
```

### versions
- `go run github.com/99designs/gqlgen version`?
v0.17.43
- `go version`?
go1.21.6 darwin/arm64