Reorder Synapse config's `experimental_features` alphabetically

roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2

@@ -2970,6 +2970,9 @@ background_updates:
     #default_batch_size: 50
 
 experimental_features:
+  {% if matrix_synapse_experimental_features_msc3266_enabled %}
+  msc3266_enabled: true
+  {% endif %}
   {% if matrix_synapse_experimental_features_msc3861_enabled %}
   msc3861:
     enabled: true
@@ -2984,8 +2987,5 @@ experimental_features:
   {% if matrix_synapse_experimental_features_msc4108_enabled %}
   msc4108_enabled: true
   {% endif %}
-  {% if matrix_synapse_experimental_features_msc3266_enabled %}
-  msc3266_enabled: true
-  {% endif %}
 
 # vim:ft=yaml