Add support for regional NEG stripping of maxUtilization based on url

[modular-magician]

Fixes: hashicorp/terraform-provider-google#7051

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Release Note Template for Downstream PRs (will be copied)

compute: Added support to `google_compute_backend_service` for setting a serverless regional network endpoint group as `backend.group`

Derived from GoogleCloudPlatform/magic-modules#3884