optimize [QDate|QTime|QDateTime]::fromString format literals. (#1278)

optimize [QDate|QTime|QDateTime]::fromString format literals. (#1278)

* optimize [QDate|QTime|QDateTime]::fromString format literals.

These accept QStringViews in Qt6 which can be very efficiently
passed with UTF-16 character literals.

* optimize QDateTime::toString format literals.

This isn't new to Qt6, but we had a few we hadn't taken care of
yet.

* use QString::replace(QChar, QChar) instead of (QString, QString)