The WebAuthn policy logic was recently merged (mrparkers/terraform-provider-keycloak#356), the logic for registering a required action hasn't been merged yet (Edit: it has been merged) (mrparkers/terraform-provider-keycloak#385), but the logic isn't required when the action is already registered (AFAIK).
Anyways, we probably need to wait for a new terraform-provider-keycloak release, before merging this.
/cc: @svenstaro