Redshift in Houdini Qt5 VS Qt6 compatibility

Our Houdini plugins are built to match Houdini's default Qt version:

Houdini 20.5 plugins work with Houdini 20.5's default Qt5 build
Houdini 21.0 plugins work with Houdini 21.0's default Qt6 build

When you download Houdini from SideFX without selecting a specific build variant, you'll get the compatible version. We won't be releasing Qt6 plugins for Houdini 20.5 or Qt5 plugins for Houdini 21.0.