3scale: Don't upgrade all connections

This is conceptually broken -- the browser should decide when it wants
to upgrade a connection to websocket, and we should just pass that
through (like we do with `Upgrade:` already). This is also what the real
3scale does, so more faithfully reproduces its behaviour.

This also helps to avoid aio-libs/aiohttp#4581

3scale/nginx.conf

@@ -51,7 +51,7 @@ http {
             proxy_http_version 1.1;
             proxy_buffering off;
             proxy_set_header Upgrade $http_upgrade;
-            proxy_set_header Connection "upgrade";
+            proxy_set_header Connection $http_connection;
 
             # Pass ETag header from Cockpit to clients.
             # See: https://github.com/cockpit-project/cockpit/issues/5239