Silence CMake deprecation warning

[christophfroehlich]

as per title, downstream packages linking against generate_parameter_library still see

CMake Deprecation Warning at /opt/ros/jazzy/share/tl_expected/cmake/tl_expectedConfig.cmake:37 (message):
  Package 'tl_expected' is deprecated (This package will be removed in ROS 2
  Lyrical Luth.

        Use tl/expected.hpp from libexpected-dev,
        or rcpputils/tl_expected/expected.hpp if the system header is not available)
Call Stack (most recent call first):
  /opt/ros/jazzy/share/parameter_traits/cmake/ament_cmake_export_dependencies-extras.cmake:21 (find_package)
  /opt/ros/jazzy/share/parameter_traits/cmake/parameter_traitsConfig.cmake:41 (include)
  /opt/ros/jazzy/share/generate_parameter_library/cmake/generate_parameter_library-extras.cmake:30 (find_package)
  /opt/ros/jazzy/share/generate_parameter_library/cmake/generate_parameter_libraryConfig.cmake:41 (include)
  CMakeLists.txt:45 (find_package)