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>