Remove the creation of the PostgreSQL single server experience

[jinglouMSFT]

The single server offering in Azure Database for PostgreSQL will be retired on March 31, 2023. We should remove the creation of the single server in the tool. Below are the schedules on the service side:

We'll announce the retirement of the Single Server offering in Azure Database for PostgreSQL by 31 March 2023. We'll continue to support customers on Single Server in maintenance mode for another two years until 31 March 2025.