mediawiki: Update maintainer description

https://phabricator.wikimedia.org/T266186 raise some questions on who is
actually maintaining this image. I think it makes more sense to describe this
as something done by MediaWiki community members rather than as by
the Wikimedia Foundation.

mediawiki/maintainer.md

@@ -1 +1 @@
-[Wikimedia Foundation & Docker Community](%%GITHUB-REPO%%)
+[MediaWiki community & Docker Community](%%GITHUB-REPO%%)