compute: promote mirror_percent in url map request_mirror_policy to GA

[eidam]

Summary

mirrorPercent in RequestMirrorPolicy is available in the Compute v1 (GA) API, but the provider gates the corresponding mirror_percent field to google-beta only in both google_compute_url_map and google_compute_region_url_map.

Evidence it is GA — the v1 discovery document defines it directly on the shared RequestMirrorPolicy schema (referenced by HttpRouteAction, used by both global and regional URL maps):

// https://www.googleapis.com/discovery/v1/apis/compute/v1/rest
"RequestMirrorPolicy": {
  "properties": {
    "mirrorPercent": { "type": "number", "format": "double", "description": "The percentage of requests to be mirrored to `backend_service`." },
    "backendService": { "type": "string" }
  }
}

REST reference: https://cloud.google.com/compute/docs/reference/rest/v1/urlMaps

The field was originally added as beta-only in #13974 (global) and #14493 (regional); this promotes it to GA.

Changes

• Remove min_version: beta from all four mirrorPercent field definitions in each of UrlMap.yaml and RegionUrlMap.yaml (default_route_action, path-matcher default_route_action, path-rule route_action, route-rule route_action).
• Remove min_version: 'beta' from the corresponding *_mirror_percent examples.
• Drop the google-beta provider bindings from those example configs so they run against the GA provider. All resources used are GA.

Other unrelated beta gating in these files is left untouched.

Release Note

compute: promoted `mirror_percent` in `request_mirror_policy` to GA for `google_compute_url_map` and `google_compute_region_url_map`

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[ollym]

Need this too, please 🙏