[BUG] Need to address MULTIPLE_DEFAULT_TAGS flagged in PostgreSQL Repo

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/main/specification/postgresql/resource-manager/readme.md

### API Spec version

2025-08-01

### Describe the bug

Need to remove use of default tag once APIs reference Single Server are deprecated and removed.

Currently receiving Approved Avocado for the pre-existing issue to bypass: https://github.com/Azure/azure-rest-api-specs/pull/39125#issuecomment-3628749751

Until we deprecate older APIs we cannot remove the single server default tag. Example if we attempt
`{"level":"Error","code":"MISSING_APIS_IN_DEFAULT_TAG","message":"The default tag should contain all APIs. The API path `/subscriptions/{}/resourcegroups/{}/providers/microsoft.dbforpostgresql/servers/{}/securityalertpolicies` is not in the default tag. Please make sure the missing API swaggers are in the default tag.","tag":"default","readMeUrl":"/home/runner/work/azure-rest-api-specs/c93b354fd9c14905bb574a8834c4d69b/specification/postgresql/resource-manager/readme.md","jsonUrl":"/home/runner/work/azure-rest-api-specs/c93b354fd9c14905bb574a8834c4d69b/specification/postgresql/resource-manager/Microsoft.DBforPostgreSQL/preview/2017-12-01-preview/postgresql.json","path":"/home/runner/work/azure-rest-api-specs/c93b354fd9c14905bb574a8834c4d69b/specification/postgresql/resource-manager/Microsoft.DBforPostgreSQL/preview/2017-12-01-preview/postgresql.json","apiPath":"/subscriptions/{}/resourcegroups/{}/providers/microsoft.dbforpostgresql/servers/{}/securityalertpolicies"}`

### Expected behavior

n/a

### Actual behavior

n/a

### Reproduction Steps

n/a

### Environment

_No response_