Move customResponseHeaders to beta only

[modular-magician]

Fixes #7299

customResponseHeaders only appears in the beta Compute API, and should be removed from the GA provider.

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• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
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• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

compute: removed `custom_response_headers` from GA `google_compute_backend_service` since it only works in the Beta version

Derived from GoogleCloudPlatform/magic-modules#4286

[ghost]

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