Use consistent bash5 style for conditionals and quoting

migrations/1757361127.sh

@@ -1,6 +1,6 @@
 echo "Fix DHCP DNS to allow VPN DNS override"
 
-if [ -f /etc/systemd/resolved.conf ]; then
+if [[ -f /etc/systemd/resolved.conf ]]; then
   if grep -q "^DNS=$" /etc/systemd/resolved.conf && grep -q "^FallbackDNS=$" /etc/systemd/resolved.conf; then
     sudo sed -i '/^DNS=$/d; /^FallbackDNS=$/d' /etc/systemd/resolved.conf
     sudo systemctl restart systemd-resolved