One of solutions for random 502 errors from caddy is to fully disable
HTTP2 protocol ( https://github.com/mholt/caddy/issues/1080 )

We run Caddy with HTTP2 enabled by default, as we can enable/disable it per
each slave, but in some environments it might be just better to fully avoid
HTTP2 codepaths in Caddy.

/reviewed-on nexedi/slapos!495