Make mastodon-streaming systemd unit templated (#24751)

Co-authored-by: Emelia Smith <ThisIsMissEm@users.noreply.github.com>

dist/nginx.conf

@@ -8,7 +8,15 @@ upstream backend {
 }
 
 upstream streaming {
+    # Instruct nginx to send connections to the server with the least number of connections
+    # to ensure load is distributed evenly.
+    least_conn;
+
     server 127.0.0.1:4000 fail_timeout=0;
+    # Uncomment these lines for load-balancing multiple instances of streaming for scaling,
+    # this assumes your running the streaming server on ports 4000, 4001, and 4002:
+    # server 127.0.0.1:4001 fail_timeout=0;
+    # server 127.0.0.1:4002 fail_timeout=0;
 }
 
 proxy_cache_path /var/cache/nginx levels=1:2 keys_zone=CACHE:10m inactive=7d max_size=1g;