Mention GODEBUG usage to enforce HTTP/1

[mstoykov]

After grafana/k6#2222 k6 now supports a way to enforce HTTP/1 using GODEBUG=http2client=0 k6 run script.js or the corresponding way for windows.

This isn't documented anywhere and we do get questions about it and it's also sometimes requires for people using NTLM and servers who let them upgrade to HTTP2 but require NTLM 🤦

I am not certain to what extent we also don't want to mark this with a warning that we might break it for future APIs.

edit: this does not work in the cloud as you can not set GODEBUG for the whole process. And that is unlikely to change.
Support for cloud will need to wait for the new HTTP API.