Update docs on @defer directive usage with GraphQL Code Generator

[beerose]

We recently added and released a @defer directive support in GraphQL Code Generator — dotansimha/graphql-code-generator#9196. This PR updates the Apollo Client documentation on the status of defer support in the Codegen.

Checklist:

• If this PR contains changes to the library itself (not necessary for e.g. docs updates), please include a changeset (see CONTRIBUTING.md)
• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

[apollo-cla]

@beerose: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

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[changeset-bot]

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Latest commit: 9ed5f47

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[alessbell]

LGTM, thanks!