fix(backend): Add suggestions from PR review

Author: lusu007

backend/templates/_listen.conf

@@ -6,16 +6,13 @@
 {% endif %}
 {% if certificate -%}
   listen 443 ssl;
-  {% if http2_support == 1 or http2_support == true %}
-  http2;
-  {% endif %}
 {% if ipv6 -%}/
   listen [::]:443 ssl;
-  {% if http2_support == 1 or http2_support == true %}
-  http2;
-  {% endif %}
 {% else -%}
   #listen [::]:443;
 {% endif %}
+{% endif %}
+{% if http2_support == 1 or http2_support == true %}
+http2 on;
 {% endif %}
   server_name {{ domain_names | join: " " }};