GREATER_EQUAL was introduced in cmake 3.7, but the specified minimum version is 2.8.11.
So use NOT LESS instead to still support older cmake versions too.

Builds fine with cmake 3.5.2 now, and also newer versions (haven't tried older versions).

Before, I got this error when building umbrello master or 19.08.0 with cmake 3.5.2:

CMake Error at CMakeLists.txt:12 (if):
  if given arguments:

    "KDE_APPLICATIONS_VERSION_MICRO" "GREATER_EQUAL" "70"

  Unknown arguments specified