[CMake] require ROOT 6.28.0 (needed for C++-17)

CMakeLists.txt

@@ -118,18 +118,12 @@ if(NOT DISABLE_LLN_MATRIX)
 endif()
 
 if(NOT DISABLE_CERN_ROOT)
-  find_package(CERN_ROOT)
+  find_package(CERN_ROOT 6.28.00) # required for C++17
   if (CERN_ROOT_FOUND)
     message(STATUS "ROOT Version: ${CERN_ROOT_VERSION}")
     if (${CERN_ROOT_VERSION} VERSION_GREATER 6.29.99)
         message(STATUS "ROOT Version is >= 6.30. Setting the minimum CXX version to 20.")
         UseCXX(20)
-    elseif (${CERN_ROOT_VERSION} VERSION_GREATER 6.27.99)
-        message(STATUS "ROOT Version is >= 6.28. Setting the minimum CXX version to 17.")
-        UseCXX(17)
-    elseif (${CERN_ROOT_VERSION} VERSION_GREATER 6.23.99)
-        message(STATUS "ROOT Version is >= 6.24. Setting the minimum CXX version to 14.")
-        UseCXX(14)
     endif()
   endif()
 endif()