QtROExternalIoDevice never connects the external QIODevice's "disconnected()" signal

In Qt 6.2.4 (and possibly earlier versions) when using an external QIODevice, the QtROExternalIoDevice wrapper should connect the QIODevice's disconnected() signal (if it exists) to its own signal. This is vital for connection management on the server side (and, as a side note, the need for a disconnected() signal in the external QIODevice-derived class should be clearly documented).

But the syntax used in the code to search for the signal (meta->indexOfSignal(SIGNAL(disconnected())))) is inadequate and always returns -1: SIGNAL prepends "2" in front of the signal signature, which makes indexOfSignal fail.

I believe the attached patch (against 6.2.4) should fix it.