StdIOClientInterface: Connect to done() signal

Instead of connecting to finished() signal.
This should also handle a failed to start case.

Change-Id: I000625adcceff94cf374cb2bfa9a994650cc261a
Reviewed-by: David Schulz <david.schulz@qt.io>
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/languageclient/languageclientinterface.h

@@ -97,7 +97,6 @@ protected:
 private:
     void readError();
     void readOutput();
-    void onProcessFinished();
 };
 
 } // namespace LanguageClient