postgres: upgrade_pg: bump major version to 16

roles/postgres/files/upgrade_pg.sh

@@ -5,7 +5,7 @@ set -e
 TO_VERSION=$(pacman -Q postgresql | grep -Po '(?<=postgresql )[0-9]+\.[0-9]')
 
 to_major=${TO_VERSION%%.*}
-if (( $to_major != 15 )); then
+if (( $to_major != 16 )); then
 	# NOTE: When this happens you should check the changelog and add any
 	# necessary changes here. Then bump the version check.
 	echo "ERROR: major upgrade detected, aborting..."