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

Start by reviewing the DBforPostgreSQL.Management API spec and compare specification/contosowidgetmanager as the v1 reference with specification/widget as the v2 reference. Wait for PR #43065 to merge, then convert the DBforPostgreSQL folder structure and verify the resulting layout matches the v2 pattern before opening a PR.

Written by the indexing model from the issue text.