Sadly nginx not support proxy to a http/2 backend server, referenced from https://www.nginx.com/blog/http2-module-nginx/#QandA
Q: Will you support HTTP/2 on the upstream side as well, or only support HTTP/2 on the client side?
A: At the moment, we only support HTTP/2 on the client side. You can’t configure HTTP/2 with proxy_pass. [Editor – In the original version of this post, this sentence was incorrectly transcribed as “You can configure HTTP/2 with proxy_pass.” We apologize for any confusion this may have caused.]
But what is the point of HTTP/2 on the backend side? Because as you can see from the benchmarks, there’s not much benefit in HTTP/2 for low‑latency networks such as upstream connections.
Also, in NGINX you have the keepalive module, and you can configure a keepalive cache. The main performance benefit of HTTP/2 is to eliminate additional handshakes, but if you do that already with a keepalive cache, you don’t need HTTP/2 on the upstream side.