Add NO_INTROSPECTION to ApolloServerErrorCode and set the code when throwing
When introspection is disabled, a `ValidationRule` is added that will throw a `GraphQLError` if an introspection request is received.   If you are logging server errors, there can be a lot of noise if someone is attempting to introspect your production servers.  To date, we have needed to filter these errors by string matching on the error message, which is not great.

Apollo Server v4 introduced the `ApolloServerErrorCode` enum.  It would be nice to see `NO_INTROSPECTION` added to this enum and then used as the error.extensions.code when you throw the `NoIntrospection` validation rule here: 
https://github.com/apollographql/apollo-server/blob/main/packages/server/src/ApolloServer.ts#L77

Perhaps add a concrete NoIntrospectionError to internalErrorClasses as well, although that doesn't help someone who would like to use an exported type to check 
```ts
if (error instanceof NoIntrospectionError)
```
