As long as Apache and OpenSSL are new enough (and they probably are with Debian or Ubuntu releases supported by Virtualmin), enabling http/2 should just be a matter of loading the http2 module and adding Protocols h2 h2c http/1.1 to the main Apache config. Make sure only one MPM module is loading, either worker or event.
The error about php_admin_value suggests the mod_php module was loading at one time but no longer is, which is what you want. See if commenting-out that line in your Apache config gets Apache server to start.
Thank you for your assistance. It gave me a level of certainty that this would not be as hard as I was worried about! I made all the changes, and it worked “out of the box”!
That’s wonderful, @NigelAves. Did you tweak settings in Virtualmin to enable HTTP/2 for just this one website / virtual server or were you able to configure Virtualmin server templates such that HTTP/2 is enabled for any new virtual servers which are created henceforth?