It’s no longer installed in the default installation. And, you probably shouldn’t use it. Apache mod_dav is not suitable for a modern shared hosting environment, from a security perspective.
If your system is not a shared system (i.e. only trusted users will have access), you can install the wbm-virtualmin-dav module, and make sure the Apache dav module is installed and enabled. But, this module is deprecated and no longer receives updates or support. There are probably better ways to provide access to files to your users (including the File Manager in Usermin/Webmin).