Change default channel from #mediawiki to #wikimedia-dev

This change configures the wikibugs IRC bot to use #wikimedia-dev as the
default destination for Bugzilla updates of products which do not have a
specific channel configured. This change should be merged together with
I10a02bfeb, which also documents the rationale for the move. (In short:
 #mediawiki is a support channel and bots are unwelcoming.)

Bug: 46144
Bug: 46282
Bug: 46322

Change-Id: Ic18d7117af94164a4b7dd951dcf53be9dd87b168