Support location in google_cloudbuild_trigger.

[modular-magician]

Summary of changes:

• Add a url-only location parameter that defaults to "global"
• The base paths have been updated to include locations/{{location}}/.
• A schema migration inserts location="global" for already existing resources. https://www.terraform.io/plugin/sdkv2/resources/state-migration
• To keep backwards compatibility in resource id, the post_create and post_import overrides change the resource id to projects/x/triggers/y instead of projects/x/locations/global/triggers/y for global triggers.
• A pre_read override is necessary to continue supporting projects/x/triggers/y as import formats.
• The schemaVersion field has been moved from resource_definition override to terraform.yaml. It has been increased to 2 to force migration for existing resources.
• The migration from V0 to V1 is noop. Migration from V1 to V2 adds location="global".
• Small fix to resource.erb template to properly handle generation of resources with more than 2 versions.
• A handwritten test verifies that a resource can be created with an old version and then updated with the current version of the provider.

Fixes hashicorp/terraform-provider-google#11624

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• 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).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
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Release Note Template for Downstream PRs (will be copied)

cloudbuild: added `location` field to `google_cloudbuild_trigger` resource

Derived from GoogleCloudPlatform/magic-modules#6440