The 2015-12-21 link on the main page of mediawiki.org, under news for maintenance updates (https://lists.wikimedia.org/pipermail/mediawiki-announce/2015-December/000187.html) points to a mailing list post which includes links to versions and corresponding GPG signatures. However, the links in that post that point to the .sig files for the full updates are all misnamed, and hence broken. Testing one of them, it seems like the "tar" suffix was dropped; for example,

https://releases.wikimedia.org/mediawiki/1.25/mediawiki-1.25.5.gz.sig

should be

https://releases.wikimedia.org/mediawiki/1.25/mediawiki-1.25.5.tar.gz.sig