Someone else may comment but we focus on helping people get and use Let's Encrypt certificates. That error is about how Apache was built and your system is configured.
The same warning message was commented on in the thread I link below. Although that was Debian not DietPi
I searched google for "this version of mod_ssl was compiled against a newer library" and got a number of suggestions. The ApacheLounge forum might be a place to ask or even on a DietPi support forum (probably better).
Just wanted to give you some suggestions for help in case no one adds further info here.