Add the option to configure external postgresql port (#20370)

While the normal assumption of port `5432` for a postgresql server is pretty reliable I found that DigitalOcean puts them on a somewhat random port. This adds the ability to specify the port in the helm chart.

chart/values.yaml

@@ -149,6 +149,7 @@ postgresql:
   # must match those of that external postgres instance
   enabled: true
   # postgresqlHostname: preexisting-postgresql
+  # postgresqlPort: 5432
   auth:
     database: mastodon_production
     username: mastodon