feat(db): use POSTGRES_USER in pg_backup; document it in release note

- pg_backup.sh: use the db service's POSTGRES_USER (default postgres) for the
  dump/drop/recreate instead of detecting the superuser at runtime, since the
  recipe now sets that env var; one-line the constants comment
- release note: explain the in-place pg_upgrade + the POSTGRES_USER override
- bump PG_BACKUP_VERSION v4

Verified on cctest: backup + restore via the hooks round-trips with POSTGRES_USER.

release/1.0.0+3.5.3

@@ -8,3 +8,13 @@ Rename these in your app's .env (the values carry over):
   DISCOURSE_SMTP_USER     --> DISCOURSE_SMTP_USER_NAME
   DISCOURSE_SMTP_AUTH     --> DISCOURSE_SMTP_AUTHENTICATION
   DISCOURSE_SMTP_PROTOCOL --> DISCOURSE_SMTP_ENABLE_START_TLS (takes a boolean true/false, not the old tls/ssl value, so translate it rather than copying it straight across)
+
+The database now runs the discourse/postgres image, which upgrades your existing
+postgres cluster to a new major version in place on first boot (no dump/restore).
+
+That upgrade runs as the cluster's bootstrap superuser ("install user"), which the
+recipe sets via POSTGRES_USER, defaulting to `postgres`. This default is correct
+for clusters created by this recipe and the old bitnami recipe, so most operators
+need to do nothing. ONLY if your cluster was bootstrapped with a different
+superuser (e.g. `discourse`) must you set POSTGRES_USER in your app's .env before
+upgrading -- otherwise pg_upgrade aborts with an install-user mismatch.