'QMap::operator[](const Key &key) const' should return 'const' value

Since Qt 6 (https://codereview.qt-project.org/c/qt/qtbase/+/297563),
QMap::operator[](const Key &key) const declaration has been changed and
began to return a non-constant value. This can lead to unexpected bugs in user code,
which are no longer detected by the compiler (as it was in Qt 5).

const QVariantMap map;
...
map["someKey"] = someValue; // now it's compiled!
...

See https://codereview.qt-project.org/c/qt/qtbase/+/297563/comment/0ed12f8d_581ffbc7/