Update references to JRegistry for administrator folder

At 3.3 when transitioning from JRegistry to `\Joomla\Registry\Registry`, none of the class references were updated as a method of demonstrating the B/C that was retained with this change through class aliasing.  The unfortunate side effect is that IDE references to the now non-existent JRegistry were broken.  This PR is the first in a series that will update the CMS's code to correctly reference the namespaced class.

To test this pull, please ensure that functions in the admin section using the Registry class still work as advertised (it looks like it's commonly used in save operations).