invalid role(s) for extra user roles (admin)

## Steps to reproduce

1. deploy `postgresql` 16/stable in a VM
2. deploy `temporal-k8s` in a k8s model
3. integrate the two

the steps above are my current deployment, but it's possible to reproduce this issue with any application that sets `extra-user-roles` to `admin` in the relation.

## Expected behavior

The application gets credentials with the `ADMIN` role from the postgresql service.

## Actual behavior

postgresql application enters a `blocked` state with the status `invalid role(s) for extra user roles`.

## Versions

Operating system: Ubuntu 24.04.2 LTS

Juju CLI: 3.6.8-genericlinux-amd64

Juju agent: 3.6.5

Charm revision: 16/stable (843)

LXD: 5.21.3 LTS

## Additional context

PG 16 charm replaced the `admin` role with `charmed_admin`. This is a breaking change, as there's no backward compatibility with existing charms that request the former.

Start by tracing how the PostgreSQL 16 charm validates the relation's extra-user-roles value and maps the legacy admin role to charmed_admin. Reproduce the integration with temporal-k8s or another application requesting admin; done means the application receives the expected ADMIN credentials without the postgresql application entering a blocked state.

Written by the indexing model from the issue text.