Thank you for the answers.
I used this script to fix my problem:
find_package(VTK 7.1.0 QUIET)
if (NOT VTK_FOUND)
find_package(VTK 8.0.0 REQUIRED)
endif()
include(${VTK_USE_FILE})
Of course this will only work until there is a VTK 9.x.x version.