Enable EXTERNAL_MANAGED resource examples for GA

[modular-magician]

One last change for the Global External HTTP(S) Load Balancer to remove the beta-only requirement for the examples and tests. This is for the EXTERNAL_MANAGED load_balancing_scheme option for google_compute_global_forwarding_rule and google_compute_backend_service.

Really fixes hashicorp/terraform-provider-google#10858

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Release Note Template for Downstream PRs (will be copied)

compute: promoted `EXTERNAL_MANAGED` value for `load_balancing_scheme` in `google_compute_backend_service ` and `google_compute_global_forwarding_rule` to GA

Derived from GoogleCloudPlatform/magic-modules#5660