dietpi-patches: replace "usermod -r" with gpasswd

"usermod -r" is available only from Bookworm on. To be compatible with Bullseye, we switch to gpasswd. It however fails, if the user is not actually member of the group anymore, hence adjust the test as well.

.update/patches

@@ -2132,7 +2132,7 @@ Patch_9_11()
 			[[ -d '/etc/nginx/sites-dietpi' ]] && G_EXEC rm -f /etc/nginx/sites-dietpi/dietpi-pihole*
 
 			# Remove www-data user from pihole group
-			getent group pihole > /dev/null && G_EXEC usermod -rG pihole www-data
+			getent group pihole | grep -Eq ':www-data(,|$)' && G_EXEC gpasswd -d www-data pihole
 
 			# Change web UI port to 8089 to avoid conflict with webservers and other web applications
 			G_EXEC pihole-FTL --config webserver.port 8089