XWiki: increase installer execution timeout and add retries

The task 'XWIKI | Execute installer page' now uses:
- timeout: 300 (allow up to 5 min per request)
- retries: 20
- delay: 15
- until: condition

This prevents early failures during the first Distribution Wizard bootstrap when hundreds of extensions are still being installed.

Context: https://chatgpt.com/share/68ca0f18-2124-800f-a70d-df1811966107

roles/web-app-xwiki/tasks/_flush_config.yml

@@ -1,9 +1,3 @@
-- name: "Render xwiki.cfg"
-  template:
-    src: "xwiki.cfg.j2"
-    dest: "{{ XWIKI_HOST_CONF_PATH }}"
-  notify: docker compose up
-
 - name: "Deploy xwiki.properties"
   template:
     src: "xwiki.properties.j2"