Remove mentioning of older Qt versions from QtC documentation

Qt Creator documentation mentions details about Qt 4 and early Qt 5 versions that are unnecessary nowadays, and might let the documentation look outdated. Take e.g. https://doc.qt.io/qtcreator/creator-qml-performance-monitor.html.

On the best cut-off date: The only somewhat still supported Qt 5 version is 5.15. If we'd like to be conservative, we could keep details about Qt 5.6 onwards. But the number of people still actively developing wiht older Qt 5 versions than this, or Qt 4, should be rather low.