VCS: Fix build with Qt6

QStringRef is gone.

Task-number: QTCREATORBUG-24098
Change-Id: I38dd1602294fcabf5cf7c1553bcfd93f3c47b601
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/cvs/cvsutils.cpp

@@ -222,7 +222,7 @@ QString previousRevision(const QString &rev)
     const int dotPos = rev.lastIndexOf(QLatin1Char('.'));
     if (dotPos == -1)
         return rev;
-    const int minor = rev.midRef(dotPos + 1).toInt();
+    const int minor = rev.mid(dotPos + 1).toInt();
     return rev.left(dotPos + 1) + QString::number(minor - 1);
 }