Fixed preconfigured_waf_config permadiff causing rules being recreated

[matheusaleixo-cit]

Fixes a permadiff related to the preconfigured_waf_config field in google_compute_security_policy. Also promotes preconfigured_waf_config to GA.

Part of: https://github.com/hashicorp/terraform-provider-google/issues/16882, https://github.com/hashicorp/terraform-provider-google/issues/18596

Release Note Template for Downstream PRs (will be copied)

compute: fixed permadiff on the `preconfigured_waf_config` field for `google_compute_security_policy` resource

compute: added `preconfigured_waf_config` field  to `google_compute_security_policy` resource (ga)