Fix crash when removing built-in snippets

Snippets collection managed the snippets with persistent iterators to
the "end" of the active snippets. Qt 5's QList was nicely updating
iterators even on non-const operations, but that has changed in Qt 6.

Don't use iterators, simply work with the count of active items.

Fixes: QTCREATORBUG-26648
Task-number: QTCREATORBUG-24098
Change-Id: Icc13274daf0e87d0562d80d7bc763c863d5da075
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/texteditor/snippets/snippetscollection.h

@@ -121,7 +121,7 @@ private:
     // active. Specifically, removed built-in snippets are kept as the last ones (for each
     // group there is a iterator that marks the logical end).
     QVector<QList<Snippet> > m_snippets;
-    QVector<QList<Snippet>::iterator> m_activeSnippetsEnd;
+    QVector<int> m_activeSnippetsCount;
 
     QHash<QString, int> m_groupIndexById;
 };