Change the default PostgreSQL version for PostgreSQL Flexible server from "12" to "15"

[v-ruizh]

OS: All
Build Version: 20231007.1

Repro Steps:

1. Create one PostgreSQL Flexible server in VS Code.
2. Check whether the PostgreSQL version for the above PostgreSQL Flexible server is "15" or not.

Expect:
The PostgreSQL version for the above Flexible server is "15", such as the PostgreSQL Flexible server created from portal.

Actual:
The PostgreSQL version for the above Flexible server is "12".

More Info:
The default PostgreSQL version is "15" for PostgreSQL Flexible server in Azure portal.

[JasonYeMSFT]

The highest version the SDK supports is 14. I will bump our default version to 14.