Change region field required to default_from_api

[modular-magician]

Change region field for google_compute_region_ssl_policy to be optional and to be inferred from environment.

Fixes hashicorp/terraform-provider-google#17142

• Searched through the issue tracker for an open issue that this either resolves or contributes.
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests). - read only field
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

compute: Changed `google_compute_region_ssl_policy`'s `region` field to optional and allow to be inferred from environment

Derived from GoogleCloudPlatform/magic-modules#10724