Remove welcome_user_id from Element and Schildichat

Ref:
- https://github.com/matrix-org/matrix-react-sdk/pull/12153
- https://github.com/element-hq/element-web/pull/26885

Technically, it may still work for Schildichat, because it's stuck in
the past. It will catch up soon anyway.

roles/custom/matrix-client-schildichat/tasks/validate_config.yml

@@ -1,5 +1,14 @@
 ---
 
+- name: (Deprecation) Catch and report renamed Schildichat settings
+  ansible.builtin.fail:
+    msg: >-
+      Your configuration contains a variable, which now has a different name.
+      Please change your configuration to rename the variable (`{{ item.old }}` -> `{{ item.new }}`).
+  when: "item.old in vars"
+  with_items:
+    - {'old': 'matrix_client_schildichat_welcome_user_id', 'new': '<removed>'}
+
 - name: Fail if required schildichat settings not defined
   ansible.builtin.fail:
     msg: >