backward compatible changes for Qt6. (#640)

backward compatible changes for Qt6. (#640)

* backward compatible changes for Qt6.

This is mostly QRegExp -> QRegularExpression.
Some QRegExp usage was was replaced by QString::trimmed.
There is also a QNetworkRequest redirect attribute change that is backward
compatible to Qt5.9.

* eliminate usage of unnecessary regular expressions.

* bring serial_unix into Qt6 compliance.

QSet::toList is gone, QSet::values is identical.
qSort is deprecated.