cmake: Add VERSION_VAR in find package module

Fixes build with latest CMake master due to changes in find_package_handle_standard_args. The package is marked as not found because a version was requested in find_package, but VERSION_VAR was not set. FATAL_ON_MISSING_REQUIRED_PACKAGES then aborts the build.

Upstream discussion https://gitlab.kitware.com/cmake/cmake/-/merge_requests/7527#note_1245246