Raise standalone default Matrix Client API client_max_body_size

We do this to match Synapse's new default "max_upload_size" (50MB).

This `matrix_nginx_proxy_proxy_matrix_client_api_client_max_body_size_mb`
default value only affects standalone usage of the `matrix-nginx-proxy`
role. When the role is used in the context of the playbook,
the value is dynamically assigned from `group_vars/matrix_servers`.

Somewhat related to #692 (Github Issue).

roles/matrix-nginx-proxy/defaults/main.yml

@@ -147,7 +147,7 @@ matrix_nginx_proxy_proxy_synapse_metrics_basic_auth_key: ""
 matrix_nginx_proxy_proxy_matrix_client_api_addr_with_container: "matrix-synapse:8008"
 matrix_nginx_proxy_proxy_matrix_client_api_addr_sans_container: "127.0.0.1:8008"
 # This needs to be equal or higher than the maximum upload size accepted by Synapse.
-matrix_nginx_proxy_proxy_matrix_client_api_client_max_body_size_mb: 25
+matrix_nginx_proxy_proxy_matrix_client_api_client_max_body_size_mb: 50
 
 
 # Tells wheter `/_synapse/client` is forwarded to the Matrix Client API server.