Mark Tails 4.2 as released.

debian/changelog

-tails (4.2) UNRELEASED; urgency=medium
+tails (4.2) unstable; urgency=medium
 
   * Major changes
     - Switch to a redesigned upgrade system (Closes: #15281), which:
@@ -60,7 +60,7 @@ tails (4.2) UNRELEASED; urgency=medium
     - Don't try to save tor control sockets as artifacts
     - Add a crude script to generate IUKs for our test suite
 
- -- Tails developers <tails@boum.org>  Mon, 06 Jan 2020 11:17:21 +0000
+ -- Tails developers <tails@boum.org>  Mon, 06 Jan 2020 16:25:22 +0000
 
 tails (4.1.1) unstable; urgency=medium