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-element/defaults/main.yml

@@ -151,7 +151,6 @@ matrix_client_element_bug_report_endpoint_url: "https://element.io/bugreports/su
 matrix_client_element_show_lab_settings: true  # noqa var-naming
 # Element public room directory server(s)
 matrix_client_element_room_directory_servers: ['matrix.org']
-matrix_client_element_welcome_user_id: ~
 # Branding of Element
 matrix_client_element_brand: "Element"
 

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

@@ -43,6 +43,7 @@
     - {'old': 'matrix_client_element_branding_authHeaderLogoUrl', 'new': 'matrix_client_element_branding_auth_header_logo_url'}
     - {'old': 'matrix_client_element_branding_welcomeBackgroundUrl', 'new': 'matrix_client_element_branding_welcome_background_url'}
     - {'old': 'matrix_client_element_jitsi_preferredDomain', 'new': 'matrix_client_element_jitsi_preferred_domain'}
+    - {'old': 'matrix_client_element_welcome_user_id', 'new': '<removed>'}
 
 - when: matrix_client_element_container_labels_traefik_enabled | bool
   block:

roles/custom/matrix-client-element/templates/config.json.j2

@@ -26,7 +26,6 @@
 	"room_directory": {
 		"servers": {{ matrix_client_element_room_directory_servers | to_json }}
 	},
-	"welcome_user_id": {{ matrix_client_element_welcome_user_id | to_json }},
 	{% if matrix_client_element_enable_presence_by_hs_url is not none %}
 		"enable_presence_by_hs_url": {{ matrix_client_element_enable_presence_by_hs_url | to_json }},
 	{% endif %}