proxy_http_version 1.0 | 1.1

 Module ngx_http_proxy_module http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_http_version

Syntax:	proxy_http_version 1.0 | 1.1;
Default:	proxy_http_version 1.0;
Context:	http, server, location

This directive appeared in version 1.1.4.

Sets the HTTP protocol version for proxying. By default, version 1.0 is used. Version 1.1 is recommended for use with keepalive connections and NTLM authentication.