Deprecate qmlmin

[ChangeLog] The qmlmin tool is deprecated and not needed anymore. The
built-in caching of qml files addresses the needs and unfortunately the
tool does not work with more advanced concepts such as required
properties or the ES 7 yield statement. It will be removed in Qt 6.

Change-Id: I2830f91d1dc2ad5e3c66270dad8e25793587ae7f
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

tools/qmlmin/main.cpp

@@ -638,6 +638,8 @@ int runQmlmin(int argc, char *argv[])
         return 0;
     }
 
+    std::cerr << "qmlmin: This tool is deprecated and will be removed in Qt 6. It is not needed anymore due to QtQml's built-in caching." << std::endl;
+
     QFile file(fileName);
     if (! file.open(QFile::ReadOnly)) {
         std::cerr << "qmlmin: '" << qPrintable(fileName) << "' no such file or directory" << std::endl;