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-synapse/tasks/ext/rest-auth/setup_install.yml

@@ -22,11 +22,15 @@
     matrix_synapse_password_providers_enabled: true
 
     matrix_synapse_container_extra_arguments: >
-      {{ matrix_synapse_container_extra_arguments|default([]) }}
-      +
-      ["--mount type=bind,src={{ matrix_synapse_ext_path }}/rest_auth_provider.py,dst={{ matrix_synapse_in_container_python_packages_path }}/rest_auth_provider.py,ro"]
+      {{
+        matrix_synapse_container_extra_arguments|default([])
+        +
+        ["--mount type=bind,src={{ matrix_synapse_ext_path }}/rest_auth_provider.py,dst={{ matrix_synapse_in_container_python_packages_path }}/rest_auth_provider.py,ro"]
+      }}
 
     matrix_synapse_additional_loggers: >
-      {{ matrix_synapse_additional_loggers }}
-      +
-      {{ [{'name': 'rest_auth_provider', 'level': 'INFO'}] }}
+      {{
+        matrix_synapse_additional_loggers
+        +
+        [{'name': 'rest_auth_provider', 'level': 'INFO'}]
+      }}