Adds a note to re-run Let’s Encrypt script on jetty upgrade.

debian/jitsi-meet-web-config.postinst

                 echo ""
                 echo "You are using jetty to serve jitsi-meet, we are now upgrading you to use nginx!"
                 echo ""
+                echo "If you are using Let’s Encrypt certificates please re-run the script."
+                echo ""
                 echo "------------------------------------------------"
                 echo ""