qt6: add version 6.9.1

[thomas]

Some packaged tools are needed to build the webengine part (aka chrome), namely node-js and python with html5lib.

[thomas]

Checked with wireshark-4.4.7 and stellarium-25.2. Stellarium shows the same well known issues as QT5 when it comes to remote OpenGL (sometimes a blank frame).
For own tests the following should be helpful:

• Environment (apply in increasing order)
  • PATH=/usr/local/qt6/bin:$PATH
  • QTDIR=/usr/local/qt6
  • CMAKE_PREFIX_PATH=/usr/local/qt6/lib/cmake
• Linking
  • e.g. CMAKE_EXE_LINKER_FLAGS='-Wl,-rpath,/usr/local/qt6/lib'
  • or patchelf --set-rpath /usr/local/qt6/lib foobar
  • and indeed export LD_LIBRARY_PATH=/usr/local/qt6/lib