fix: Change net-glb serve_while_stale type to number

[tobbbles]

In the provider, the serve_while_stale attribute is listed as an integer, https://github.com/hashicorp/terraform-provider-google/blob/main/google/resource_compute_backend_service.go#L373-L374

And also documented here as such: https://cloud.google.com/cdn/docs/serving-stale-content

Currently it is not possible to configure cdn_policy with this module and specify a serve_whilst_stale value.

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[juliocc]

thanks @tobbbles! good catch.