TLS Documentation for PostgreSQL 16 mentions old certificates interface

## Steps to reproduce

1. https://canonical-charmed-postgresql.readthedocs-hosted.com/16/how-to/enable-tls/
2. `juju integrate postgresql:certificates self-signed-certificates:certificates`

I found mention of this on the [Modern PostgreSQL charm](https://canonical-charmed-postgresql.readthedocs-hosted.com/16/explanation/charm-versions/modern-charm/) page, so it sounds like `peer-certificates` is the appropriate endpoint for enabling in-cluster TLS now?

## Expected behavior
In-cluster TLS enabled

## Actual behavior
`Unable to create integration, got error: application "postgresql" has no "certificates" relation`

## Versions

Juju CLI: 3.6.9

Juju agent: 3.6.9

Charm revision: 843