Fix deprecation warning

[Nicals]

Had this warning when compiling:

src/xplaneudpclient.cpp:87:28: warning: ‘QByteArray& QByteArray::append(const QString&)’ is deprecated: Use QString's toUtf8(), toLatin1() or toLocal8Bit() [-Wdeprecated-declarations]
   87 |         data.append(dataRef)

It seems that the data QByteArray only contains ASCII chars, so converting the original QString to the local 8bit representation should be good.

I'm done for now, have fun :)