Convert DBforPostgreSQL spec from "folder structure v1" to "folder structure v2"

### API Spec link https://github.com/Azure/azure-rest-api-specs/tree/main/specification/postgresql/DBforPostgreSQL.Management ### API Spec version 2026-04-01-preview ### Please describe the feature. Need to convert to V2 folder structure use folder specification/contosowidgetmanager as a gold-standard reference example for v1 use folder specification/widget as a gold-standard reference example for v2 create a PR converting DBforPostgreSQL from "folder structure v1" to "folder structure v2" Have this occur after the merge of https://github.com/Azure/azure-rest-api-specs/pull/43065