Upgrade Element (1.8.2 -> 1.8.4) to patch a security issue

See:
- https://github.com/vector-im/element-web/releases/tag/v1.8.4
- https://matrix.org/blog/2021/09/10/pre-disclosure-upcoming-critical-fix-for-several-popular-matrix-clients

roles/matrix-client-element/defaults/main.yml

@@ -3,7 +3,7 @@ matrix_client_element_enabled: true
 matrix_client_element_container_image_self_build: false
 matrix_client_element_container_image_self_build_repo: "https://github.com/vector-im/riot-web.git"
 
-matrix_client_element_version: v1.8.2
+matrix_client_element_version: v1.8.4
 matrix_client_element_docker_image: "{{ matrix_client_element_docker_image_name_prefix }}vectorim/element-web:{{ matrix_client_element_version }}"
 matrix_client_element_docker_image_name_prefix: "{{ 'localhost/' if matrix_client_element_container_image_self_build else matrix_container_global_registry_prefix }}"
 matrix_client_element_docker_image_force_pull: "{{ matrix_client_element_docker_image.endswith(':latest') }}"