Set url_mask optional for network endpoint group

[modular-magician]

Fixes #11242
Replaces hashicorp/terraform-provider-google-beta#4161

This argument isn't required - the gcloud command correctly creates a
serverless NEG with an API gateway resource without url_mask specified.

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Release Note Template for Downstream PRs (will be copied)

compute: fixed url_mask required mis-annotation in `google_compute_region_network_endpoint_group`, making it optional

Derived from GoogleCloudPlatform/magic-modules#5883

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