Fix build with cmake < 3.7

`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.

Test Plan:
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
```

Patch provided by: #wbauer
Reviewed By: #habacker
Differential Revision: https://phabricator.kde.org/D23635