Make TestAccComputeNetworkAttachment_networkAttachmentInstanceUsageExample be beta-only

[modular-magician]

This is to address this test failure in the GA nightly tests

------- Stdout: -------
=== RUN   TestAccComputeNetworkAttachment_networkAttachmentInstanceUsageExample
=== PAUSE TestAccComputeNetworkAttachment_networkAttachmentInstanceUsageExample
=== CONT  TestAccComputeNetworkAttachment_networkAttachmentInstanceUsageExample
    vcr_utils.go:152: Step 1/2 error: Error running pre-apply refresh: exit status 1
        Error: Unsupported argument
          on terraform_plugin_test.tf line 40, in resource "google_compute_instance" "default":
          40:         network_attachment = google_compute_network_attachment.default.self_link
        An argument named "network_attachment" is not expected here.
--- FAIL: TestAccComputeNetworkAttachment_networkAttachmentInstanceUsageExample (0.38s)
FAIL

Release Note Template for Downstream PRs (will be copied)

Derived from GoogleCloudPlatform/magic-modules#11113