install: Pin new apt-based installs to the latest postgresql.

Since we now support Postgres versions from 10 to 12, we might as well
have new installations start on Postgres 12 to avoid unnecessary
migration/upgrade work.

scripts/lib/install

@@ -292,6 +292,18 @@ EOF
 auto_renew = yes
 EOF
     fi
+
+    if [ -e "/etc/init.d/postgresql" ]; then
+        case " $os_id $os_id_like " in
+            *' debian '*)
+                cat <<EOF
+
+[postgresql]
+version = 12
+EOF
+                ;;
+        esac
+    fi
 ) > /etc/zulip/zulip.conf
 
 case ",$PUPPET_CLASSES," in