r/postgresql-server: Add script to upgrade database

The `postgresql-upgrade.sh` script arranges to run `pg_upgrade` after a
major PostgreSQL version update.  It's scheduled by a systemd unit,
_postgresql-upgrade.service_, which runs only after an OS update.

roles/postgresql-server/files/postgresql-upgrade.service

@@ -0,0 +1,18 @@
+[Unit]
+Description=Upgrade PostgreSQL database cluster
+Before=postgresql.service
+Before=systemd-update-done.service
+After=local-fs.target
+RequiresMountsFor=/var/lib/pgsql
+DefaultDependencies=no
+ConditionNeedsUpdate=/var
+
+[Service]
+Type=oneshot
+User=postgres
+ExecStart=+/usr/local/libexec/postgresql-upgrade
+Environment=PG_COLOR=always
+WorkingDirectory=~
+
+[Install]
+RequiredBy=postgresql.service