Add a preinst script to check version of postgresql

debian/gitlab.preinst

@@ -0,0 +1,14 @@
+#!/bin/sh
+
+set -e
+
+pg_version=$(pg_lsclusters |grep 5432 |awk '{print $1}')
+
+if [ "${pg_version}" -lt 12 ]; then
+  echo "========================================================================"
+  echo "Upgrade postgresql to at least version 12 before upgrading gitlab."
+  echo "You can use pg_upgradecluster command to upgrade the postgresql version."
+  echo "Aborting gitlab installation..."
+  echo "========================================================================"
+  exit 1
+fi