Run Element Web in tightened/read-only mode without a custom nginx config

Newer Element Web versions allow for the nginx port to be
overriden, etc., and provide instructions for running in read-only mode.

This makes our custom `nginx.conf` patches unnecessary.

Passing the correct `ELEMENT_WEB_PORT` environment variable
also helps with future changes.

Another benefit of this (besides keeping closer to upstream
recommendations and the improved simplicity) is that:

- the container can run its entrypoint env-substitutions code now,
  without reporting errors

- IPv6 for nginx works, so `matrix-client-element:8080` is accessible
  via IPv6 on the container network now
  (this affects only for Traefik's communicaton with Element Web
  internally; public connectivity was handled by Traefik and IPv6 was
  available there even before)

Ref:

- 2052080d7d/docs/install.md (docker)
- https://github.com/element-hq/element-web/pull/28849
- https://github.com/element-hq/element-web/pull/28840

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

@@ -95,12 +95,17 @@
     owner: "{{ matrix_user_username }}"
     group: "{{ matrix_user_groupname }}"
   with_items:
-    - {src: "{{ role_path }}/templates/nginx.conf.j2", name: "nginx.conf"}
     - {src: "{{ role_path }}/templates/labels.j2", name: "labels"}
+    - {src: "{{ role_path }}/templates/env.j2", name: "env"}
     - {src: "{{ matrix_client_element_page_template_welcome_path }}", name: "welcome.html"}
     - {src: "{{ matrix_client_element_embedded_pages_home_path }}", name: "home.html"}
   when: "item.src is not none"
 
+- name: Ensure Element Web nginx.conf file is removed
+  ansible.builtin.file:
+    path: "{{ matrix_client_element_data_path }}/nginx.conf"
+    state: absent
+
 - name: Ensure Element Web config files removed
   ansible.builtin.file:
     path: "{{ matrix_client_element_data_path }}/{{ item.name }}"