Special:Version: Treat AUTHORS.txt and CREDITS.txt as text, not wikitext

A review of existing extensions in git reveals that 3 of the 5
extensions that have a non-wikitext authors/credits file have this file
named AUTHORS.txt, while none of the 8 that have wikitext
authors/credits have a .txt extension. So let's fix the 2 that have
non-wikitext as "AUTHORS" and do this for the benefit of the other three
that presumably don't want to use wikitext in authors/credits.

Bug: 62153
Change-Id: Id95852ab9836fb284becd7e251374f672b69515f