Updates for com_joomlaupdate

This fixes issues raised in #5308
Renaming the XMLs is going to cause whomever is managing the releases to have even more headaches (2 XMLs per "stream" just to match a change in verbiage?).  Plus, if the old XMLs aren't maintained, users on pre-existing sites will lose their ability to pull current updates.  IMO, the comments (or future discussion) of the XML file names aren't in scope of this PR.