Update mentions of `pyqt5` inclusion as the "default" backend to `pyqt6`

[TimMonko]

🧰 Task

In napari, 0.7.0, the default qt backend included in napari[all] or napari[qt] is pyqt6, instaed of the previous pyqt5. We need to update references through the docs.

e.g. https://napari.org/stable/plugins/building_a_plugin/best_practices.html#don-t-include-napari-all-pyside2-pyqt5-or-pyqt6-in-your-plugin-s-default-dependencies

(sort of side note, but the section header below that one is rendering it as a code block and sohuld be fixed https://napari.org/stable/plugins/building_a_plugin/best_practices.html#don-t-import-from-any-specific-qt-backend-e-g-pyqt5-pyside2-etc-in-your-plugin-use-qtpy)