[FEATURE REQ] Update to use PostgreSQL flexible server in passwordless docs

[Netyyyy]

Is your feature request related to a problem? Please describe. PostgreSQL flexible server has been GAed. https://learn.microsoft.com/en-us/azure/postgresql/flexible-server/concepts-azure-ad-authentication. We should update all our usage of PostgreSQL in passwordless docs from single server to flexible server.

Describe the solution you'd like

• Update the quickstart docs
  • JAVA
  • JDBC
  • JPA
• Update the migration doc
• Update the tutorial doc
• Update the reference doc
• Update the README files in sdk/jdbc/

Describe alternatives you've considered N/A

Additional context N/A

Information Checklist

• [x] Description Added
• [x] Expected solution specified