reduce keepalive timeout

These aren't websites browsed by users. Nearly all of the requests are
done automatically in short batches other than users manually updating
or installing apps which is incredibly rare compared to the automated
requests.

nginx/nginx.conf

@@ -153,7 +153,7 @@ http {
 
         include root_releases.grapheneos.org.conf;
 
-        keepalive_timeout 3m;
+        keepalive_timeout 15s;
 
         open_file_cache max=2048 inactive=1d;
         open_file_cache_valid 1d;
@@ -286,7 +286,7 @@ http {
 
         include root_apps.grapheneos.org.conf;
 
-        keepalive_timeout 3m;
+        keepalive_timeout 15s;
 
         open_file_cache max=2048 inactive=1d;
         open_file_cache_valid 1d;