Marks template.volumes.secret.items.mode field not required in Cloud Run V2 resources

[modular-magician]

I think this field was marked as required initially by mistake. Our API doesn't require this field and doesn't set a service side default value.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• 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).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
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Release Note Template for Downstream PRs (will be copied)

cloudrunv2: Changed `template.volumes.secret.items.mode` field in `google_cloud_run_v2_job` resource to a non-required field.

cloudrunv2: Changed `template.volumes.secret.items.mode` field in `google_cloud_run_v2_service` resource to a non-required field.

Derived from GoogleCloudPlatform/magic-modules#8771