Fix compatibility with ansible=6 / ansible-core=2.13

Details here: https://docs.ansible.com/ansible/devel/porting_guides/porting_guide_6.html#id36

Basically:

```yaml
- name: Prior to 2.13
  debug:
    msg: '[1] + {{ [2] }}'

- name: 2.13 and forward
  debug:
    msg: '{{ [1] + [2] }}'
```

Interestingly, we had been using the new/safe syntax in lofs of places.

We were using the broken one in many others though. Hopefully all
instances were fixed by this patch.

roles/matrix-bridge-appservice-slack/tasks/init.yml

@@ -21,14 +21,18 @@
 # If the matrix-synapse role is not used, these variables may not exist.
 - set_fact:
     matrix_synapse_container_extra_arguments: >
-      {{ matrix_synapse_container_extra_arguments|default([]) }}
-      +
-      ["--mount type=bind,src={{ matrix_appservice_slack_config_path }}/slack-registration.yaml,dst=/matrix-appservice-slack-registration.yaml,ro"]
+      {{
+        matrix_synapse_container_extra_arguments|default([])
+        +
+        ["--mount type=bind,src={{ matrix_appservice_slack_config_path }}/slack-registration.yaml,dst=/matrix-appservice-slack-registration.yaml,ro"]
+      }}
 
     matrix_synapse_app_service_config_files: >
-      {{ matrix_synapse_app_service_config_files|default([]) }}
-      +
-      {{ ["/matrix-appservice-slack-registration.yaml"] }}
+      {{
+        matrix_synapse_app_service_config_files|default([])
+        +
+        ["/matrix-appservice-slack-registration.yaml"]
+      }}
   when: matrix_appservice_slack_enabled|bool
 
 # If the matrix-synapse role is not used, `matrix_synapse_role_executed` won't exist.