Make handleSelect=false by default and deprecate StandardRequestHandler

In the solrconfig.xml, we have a long explanation on the legacy <requestDispatcher handleSelect="false" > section. Since we are cleaning up legacy stuff for version 5, is it safe now to flip handleSelect's default to be false and therefore remove both the attribute and the whole section explaining it?

Then, a section in Reference Guide or even a blog post can explain what to do for the old clients that still need it. But it does not seem to be needed anymore for the new users. And possibly cause confusing now that we have implicit, explicit and overlay handlers.