Feature: Replace PostgreSQL Server by PostgreSQL Flexible Server

[hocinehacherouf]

Is your feature request related to a problem? Please describe.
PostgreSQL Server is deprecated and will be removed from Azure by March 28, 2025. See https://learn.microsoft.com/en-us/azure/postgresql/single-server/whats-happening-to-postgresql-single-server

As replacement, Microsoft recommends to migrate to PostgreSQL Flexible Server.

Describe the solution you'd like

1. Update bicep templates to deploy a PostgreSQL flexible server instead of a a PostgreSQL server.
2. The azure deployment (bicep templates) should be able to migrate an exisitng PostgreSQL server to an new PostgreSQL flexible server.