Cannot deploy more than 10 units (max_wal_senders limit)

[Same issue in the K8s](https://github.com/canonical/postgresql-k8s-operator/issues/895)

## Steps to reproduce

1. `juju deploy postgresql --channel edge --config profile=testing -n10` and wait them to stabilize
2. `juju add-unit postgresql`

## Expected behavior
Unit is added successfully therefore cluster consists of 11 units.

## Actual behavior

New unit stucks in `awaiting for member to start`.

## Versions

Operating system: Ubuntu 24.04.2 LTS

Juju CLI: 3.6.4-genericlinux-amd64

Juju agent: 3.6.4

Charm revision: 576

LXD: 6.3

## Log output

```
2025-03-19T20:30:09Z systemd[1]: snap.charmed-postgresql.patroni.service: Consumed 2.316s CPU time.
2025-03-19T20:30:09Z systemd[1]: Started Service for snap application charmed-postgresql.patroni.
2025-03-19T20:30:13Z charmed-postgresql.patroni[223680]: pg_basebackup: error: connection to server at "10.88.216.188", port 5432 failed: FATAL: number of requested standby connections exceeds max_wal_senders (currently 10)
2025-03-19T20:30:13Z charmed-postgresql.patroni[223680]: pg_basebackup: removing data directory "/var/snap/charmed-postgresql/common/var/lib/postgresql"
2025-03-19T20:30:20Z charmed-postgresql.patroni[223681]: pg_basebackup: error: connection to server at "10.88.216.188", port 5432 failed: FATAL: number of requested standby connections exceeds max_wal_senders (currently 10)
2025-03-19T20:30:20Z charmed-postgresql.patroni[223681]: pg_basebackup: removing data directory "/var/snap/charmed-postgresql/common/var/lib/postgresql"
2025-03-19T20:30:23Z charmed-postgresql.patroni[223683]: pg_basebackup: error: connection to server at "10.88.216.188", port 5432 failed: FATAL: number of requested standby connections exceeds max_wal_senders (currently 10)
2025-03-19T20:30:23Z charmed-postgresql.patroni[223683]: pg_basebackup: removing data directory "/var/snap/charmed-postgresql/common/var/lib/postgresql"
2025-03-19T20:30:31Z charmed-postgresql.patroni[223778]: pg_basebackup: error: connection to server at "10.88.216.188", port 5432 failed: FATAL: number of requested standby connections exceeds max_wal_senders (currently 10)
2025-03-19T20:30:31Z charmed-postgresql.patroni[223778]: pg_basebackup: removing data directory "/var/snap/charmed-postgresql/common/var/lib/postgresql"
```

## Additional context

I think including `max_wal_senders` and `max_replication_slots` config options will resolve this issue and add an abiltiy to include charm checks based on that value.