Remove qt6-base-dev from package.xml

[sloretz]

Requires http://github.com/ros/rosdistro/pull/50844 - which moves qt6-base-dev into the rosdep key python3-qt-bindings so that we can have a consistent Qt version on Ubuntu Noble.

Fixes https://build.ros2.org/view/Rbin_uN64/job/Rbin_uN64__qt_gui_cpp__ubuntu_noble_amd64__binary/118/console

14:07:52 CMake Warning at /opt/ros/rolling/share/python_qt_binding/cmake/sip_helper.cmake:38 (message):
14:07:52   Could not determine PyQt6 bindings directory.
14:07:52 Call Stack (most recent call first):
14:07:52   /opt/ros/rolling/share/python_qt_binding/cmake/sip_helper.cmake:79 (__find_qt_sip_files)
14:07:52   src/qt_gui_cpp_sip/CMakeLists.txt:71 (include)
14:07:52 
14:07:52 
14:07:52 CMake Error at /opt/ros/rolling/share/python_qt_binding/cmake/sip_helper.cmake:81 (message):
14:07:52   PyQt6 SIP bindings are required but were not found.
14:07:52 Call Stack (most recent call first):
14:07:52   src/qt_gui_cpp_sip/CMakeLists.txt:71 (include)

[sloretz]

CI (Build up to qt_gui_cpp, test python_qt_binding)

• Linux
• Linux-aarch64
• Linux-rhel
• Windows

[mini-1235]

This assumes that qt6 isn't already installed on the system, or that there aren't any other packages in the workspace pulling in qt6, correct? Otherwise, the logic in that sip_helper.cmake file would still resolve to Qt6 and trigger a build error(?)

Out of curiosity, why not make qt6 the default here? Is it because some binaries are missing in noble?