Postgresql Flexible Server should return error to not allow HA when create mode is PITR

Postgresql Flexible Server doesn't allow HA when create mode is PITR. But currently API doesn't return any error. Service team confirmed they will add this request adding validation for this situation to their plan.

No file, operation, or test is named. Start by locating the PostgreSQL Flexible Server create operation in the REST API specifications and inspect how PITR and HA are represented; done means the specification captures an error for requests combining PITR create mode with HA, subject to the service team's planned validation.

Written by the indexing model from the issue text.