Switch mautrix-slack double-puppeting method (shared secret auth -> appservice-double-puppet)

Since upgrading mautrix-slack (and pinning to v0.1.0) in e4b54c37fe,
we expect double-puppeting to require the new appservice double-puppeting method.

This commit switches the mautrix-slack bridge to it.

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

@@ -20,3 +20,14 @@
       To resolve the conflict, make one of these components use a different username.
       Consider either changing `matrix_mautrix_slack_appservice_bot_username` (the bot username for the mautrix-slack component) or `matrix_appservice_slack_bot_name` (the bot username for the appservice-slack component).
       We recommend that you change the username for the newly-added (and yet unused) component.
+
+- 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_slack_login_shared_secret', 'new': '<removed>'}
+    - {'old': 'matrix_mautrix_slack_bridge_login_shared_secret_map', 'new': '<superseded by matrix_mautrix_slack_double_puppet_secrets>'}
+    - {'old': 'matrix_mautrix_slack_bridge_double_puppet_server_map', 'new': '<removed>'}