Update synapse (1.7.2 -> 1.7.3)

roles/matrix-synapse/defaults/main.yml

@@ -3,7 +3,7 @@
 
 matrix_synapse_enabled: true
 
-matrix_synapse_docker_image: "matrixdotorg/synapse:v1.7.2"
+matrix_synapse_docker_image: "matrixdotorg/synapse:v1.7.3"
 matrix_synapse_docker_image_force_pull: "{{ matrix_synapse_docker_image.endswith(':latest') }}"
 
 matrix_synapse_base_path: "{{ matrix_base_data_path }}/synapse"