Fix Qt CMake policy

[Arthoni]

Named CMP0020, mode OLD is deprecated : https://cmake.org/cmake/help/latest/policy/CMP0020.html

Warning during precompilation:

-- Found Qt6 (version 6.2.4)
CMake Deprecation Warning at _build/dist/lib/openfluid/cmake/OpenFLUIDDetectQt.cmake:70 (CMAKE_POLICY):
  The OLD behavior for policy CMP0020 will be removed from a future version
  of CMake.

  The cmake-policies(7) manual explains that the OLD behaviors of all
  policies are deprecated and that a policy should be set to OLD only under
  specific short-term circumstances.  Projects should be ported to the NEW
  behavior and not rely on setting a policy to OLD.
Call Stack (most recent call first):
  src/openfluid/ui/waresdev/CMakeLists.txt:1 (INCLUDE)