This was announced last January.
We will continue to build against Qt 5.15.2 and are looking forward to migrating to Qt 6.0 once all the modules are available.
At present there are a significant number of important modules missing in Qt 6.0 (charts, multimedia, web) and I don't expect them to be even close to ready before we release in the summer. Honestly, I'd be surprised if there in by the end of 2021.
It means we won't be able to get bug fixes for Qt 5.15 but its pretty stable anyway.
Mark