Fix path_matcher in region url maps

[modular-magician]

The default_service in path_matcher is actually not required as it should be default_service or default_url_redirect.
(related to hashicorp/terraform-provider-google#7951)

Further info in https://cloud.google.com/compute/docs/reference/rest/v1/regionUrlMaps , extract below:

pathMatchers[].defaultService
Only one of defaultService, defaultUrlRedirect or defaultRouteAction.weightedBackendService must be set.

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google_compute_region_url_map: removed requirement for default_service, as it should be a choice of default_service or default_url_redirect

Derived from GoogleCloudPlatform/magic-modules#4298