Use liberty-eol tag for liberty release notes

Reno doesn't currently handle missing branches so we need to use the
proper tag in order to prevent releasenotes jobs from failing now that
liberty is EOL'd.

Change-Id: I5bec207ba1df40c1800c615ca5ad3a672f893648
Related-bug: #1649982

releasenotes/source/liberty.rst

@@ -3,4 +3,4 @@ Liberty Series Release Notes
 ============================
 
 .. release-notes::
-   :branch: origin/stable/liberty
+   :branch: liberty-eol