Drop Ubuntu codenames that we don't manage to maintain (refs: #12152)

Instead, link to a place that has up-to-date info.

wiki/src/contribute/release_process/tails-installer.mdwn

@@ -64,9 +64,8 @@ targeted at current Tails, as said above. More specifically:
 * The `upstream/4.x+dfsg`, `upstream/5.x+dfsg`, etc. branches are what
   we tell `gbp` to use as its "upstream" branch. Make sure to check
   them out when setting up the repository for the first time.
-* For Ubuntu, we want to support the current Ubuntu version (currently
-  `yakkety`), the upcoming version (currently `zesty`), and the
-  current LTS (currently `xenial`).
+* For Ubuntu, we want to support the current Ubuntu version, the
+  upcoming version, and the current LTS: [[!wikipedia Ubuntu version history]].
   We do not maintain any Git branches related to Ubuntu releases, as
   simply the changelog entries are modified.