In order to migrate to Qt6 all options for KF6 were added:

a) using a system provided KF6
b) downloading and compiling KF6
c) using the vendored-in re-creation of the functionality

The behaviour for both Qt5 and Qt6 is steered with the same two variables which were renamed to become version agnostic:
a) BUILD_WITH_KF=ON BUILD_KF=OFF
b) BUILD_WITH_KF=ON BUILD_KF=ON
c) BUILD_WITH_KF=OFF

The version is automatically derived from the chosen Qt major version.

tests/catch/CMakeLists.txt

@@ -90,7 +90,7 @@ add_executable(
     ${QET_DIR}/sources/borderproperties.h
     )
 
-if(NOT BUILD_WITH_KF5)
+if(NOT BUILD_WITH_KF)
     target_sources(
         ${PROJECT_NAME}
         PRIVATE
@@ -109,7 +109,7 @@ target_link_libraries(
     PUBLIC
     Catch2::Catch2
     PRIVATE
-    ${KF5_PRIVATE_LIBRARIES}
+    ${KF_PRIVATE_LIBRARIES}
     ${QET_PRIVATE_LIBRARIES})
 
 if (DEFINED ENV{CATCH_INCLUDE_DIR})

tests/googlemock/CMakeLists.txt

@@ -85,6 +85,6 @@ target_link_libraries(
     PUBLIC
     gmock gmock_main
     PRIVATE
-    ${KF5_PRIVATE_LIBRARIES}
+    ${KF_PRIVATE_LIBRARIES}
     ${QET_PRIVATE_LIBRARIES})
 

tests/googletest/CMakeLists.txt

@@ -84,6 +84,6 @@ target_link_libraries(
     PUBLIC
     gtest gtest_main
     PRIVATE
-    ${KF5_PRIVATE_LIBRARIES}
+    ${KF_PRIVATE_LIBRARIES}
     ${QET_PRIVATE_LIBRARIES})
 

tests/qttest/CMakeLists.txt

@@ -74,6 +74,6 @@ target_link_libraries(
   ${PROJECT_NAME}
   PRIVATE
   Qt::Test
-  ${KF5_PRIVATE_LIBRARIES}
+  ${KF_PRIVATE_LIBRARIES}
   ${QET_PRIVATE_LIBRARIES})