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[bot]]

Docker Image for build 5 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

[jc21]

Closing in favour of #4106 that addresses these comments