mpm_prefork is required for modperl2. There really is not other option. So until we remove the dependence on modperl2 we are stuck with it.
If you disable the http2 module, that warning will go away. It is not used by webwork2, and as long as you are using mpm_prefork, you can't use http2.
Webwork3 won't depend on modperl2, so it will be better to use mpm_event module instead with apache2.