Update matthiasluedtke/iconserver Docker tag to v3.21.0

This MR contains the following updates:

Package	Update	Change
matthiasluedtke/iconserver	minor	v3.16.0 -> v3.21.0

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webserver/docker_workadventure/templates/docker-compose.yml

@@ -77,7 +77,7 @@ services:
 
 
   icon:
-    image: matthiasluedtke/iconserver:v3.16.0
+    image: matthiasluedtke/iconserver:v3.21.0
     restart: always
     labels:
       - traefik.enable=true