Add appservice-double-puppet double-puppeting support to mautrix-discord

Shared Secret Auth double puppeting still works for this bridge, but
is deprecated and will go away in the future.

roles/custom/matrix-bridge-mautrix-discord/tasks/validate_config.yml

@@ -12,3 +12,12 @@
     - {'name': 'matrix_mautrix_discord_homeserver_public_address', when: true}
     - {'name': 'matrix_mautrix_discord_container_network', when: true}
     - {'name': 'matrix_mautrix_discord_database_hostname', when: "{{ matrix_mautrix_discord_database_engine == 'postgres' }}"}
+
+- name: (Deprecation) Catch and report renamed 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_mautrix_discord_login_shared_secret', 'new': '<removed>'}