Replace deprecated qAsConst with std::as_const

qAsConst was deprecated in Qt 6.6; std::as_const (C++17, already the
project standard) is the drop-in replacement. Clears 46 -Wdeprecated-
declarations warnings across 18 files. No behavioural change.

sources/qetproject.cpp

@@ -2029,7 +2029,7 @@ void QETProject::updateDiagramsFolioData()
 		}
 	}
 
-	for (const auto &diagram_ : qAsConst(m_diagrams_list)) {
+	for (const auto &diagram_ : std::as_const(m_diagrams_list)) {
 		diagram_->update();
 	}
 }