[cleanup] Deprecate pagegenerators.UnconnectedPageGenerator
[cleanup] Deprecate pagegenerators.UnconnectedPageGenerator

Use Site.unconnected_pages() instead

Change-Id: Icca149237b7c1385502707add11d9b359aa8c1ed

PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core,wikimedia/pywikibot-core