fix(backend): Replace deprecated http2 directive

[lusu007]

Fixes #4060

This PR replaces the listen ... http2 directive with the new and not deprecated directive http2 on. This prevents warnings in the log output.

[nginxproxymanagerci]

Docker Image for build 3 is available on
DockerHub
as nginxproxymanager/nginx-proxy-manager-dev:pr-4075

Note: ensure you backup your NPM instance before testing this image! Especially if there are database changes
Note: this is a different docker image namespace than the official image