cmake: fix eigen cmake packages for v5.0 (3.5)
Eigen changed from 3.x versioning to semver (3.5 became 5.0) and
dropped FindEigen3.cmake module support (deprecated in prior versions)
for config discovery.
Switch from find_package version constraints to manual version checking
to maintain compatibility with both versioning schemes (>= 3.3 old or
>= 5.0 new).
See: https://gitlab.com/libeigen/eigen/-/merge_requests/485
Part-of: <https://gitlab.freedesktop.org/monado/monado/-/merge_requests/2583>