chore(deps): update ghcr.io/open-webui/open-webui docker tag to v0.6.21

docker-compose/openwebui/compose.yaml

@@ -1,7 +1,7 @@
 ---
 services:
   openwebui:
-    image: ghcr.io/open-webui/open-webui:v0.6.18
+    image: ghcr.io/open-webui/open-webui:0.6.21
     container_name: openwebui
     ports:
       - "8080:8080"