Make use of the new --exists-ok flag for register_new_matrix_user

Related to https://github.com/element-hq/synapse/pull/17304

roles/custom/matrix-user-creator/tasks/util/ensure_user_registered_synapse.yml

@@ -16,7 +16,7 @@
           --user_type={{ user.initial_type | quote }}
         {% endif %}
       {% endif %}
+      --exists-ok
       http://localhost:{{ matrix_synapse_container_client_api_port }}
   register: matrix_synapse_register_user_result
-  changed_when: matrix_synapse_register_user_result.rc == 0 and 'User ID already taken' not in matrix_synapse_register_user_result.stdout
-  failed_when: matrix_synapse_register_user_result.rc != 0 and 'User ID already taken' not in matrix_synapse_register_user_result.stdout
+  changed_when: matrix_synapse_register_user_result.rc == 0 and 'User already exists' not in matrix_synapse_register_user_result.stdout