Changing preferred Qt binding from pyqt5 to qt6

[t20100]

Qt5 has reaching the end of regular support mid-2023 and only receives updates under extended commercial support silx issues occuring with Qt5 but not Qt6 starts to be reported (see #4135).

IMO, it's timely to consider changing the default Qt binding used by silx, this means:

• Changing the priorities of the different Qt bindings. Current priorities: https://github.com/silx-kit/silx/blob/a7da2634a2a26e1b77ea3e4bdf080027da2a7c0a/src/silx/gui/qt/_qt.py#L67
• Changing the Qt binding installed with pip install silx[full]

Support of PyQt5 will remain, it will just no longer be selected by default.

The following question is: PySide6 or PyQt6 ? I'm +1for PySide6.