Adjust default value for `matrix_synapse_admin_configuration_extension_json_auto` to match what we expect and how we use it

We expect a string, which is parsed via `from_json`.

This matches previous usage in inventories, as well as what the
user-controlled precedessor of this variable (`matrix_synapse_admin_configuration_extension_json`)
is like.

Related to 997e6ca611

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/4459

roles/custom/matrix-synapse-admin/defaults/main.yml

@@ -187,7 +187,7 @@ matrix_synapse_admin_configuration_default:
 matrix_synapse_admin_configuration_extension_json: '{}'
 
 # This is similar to `matrix_synapse_admin_configuration_extension_json`, but intended for use by playbook or group vars
-matrix_synapse_admin_configuration_extension_json_auto: {}
+matrix_synapse_admin_configuration_extension_json_auto: '{}'
 
 matrix_synapse_admin_configuration_extension: "{{ matrix_synapse_admin_configuration_extension_json_auto | from_json | combine(matrix_synapse_admin_configuration_extension_json | from_json if matrix_synapse_admin_configuration_extension_json | from_json is mapping else {}, recursive=True) }}"