changelogs: Mark version 20.4.0 and 20.8.0 do not use

Problem
We have two versions in the past are released with corrupt jars.

Solution
With patch released fixed the issue, we want to mark the problematic
releases to avoid usage.

JIRA Issues: CSL-10338

Differential Revision: https://phabricator.twitter.biz/D579740

CHANGELOG.rst

@@ -38,8 +38,8 @@ No Changes
 
 * scrooge-generator: Reduced the size of generated Scala code. ``PHAB_ID=D527920``
 
-20.8.0
-------
+20.8.0 (DO NOT USE)
+-------------------
 
 * scrooge: replace deprecated ScalaTest 3.0 API with ScalaTest 3.1/3.2 API. ``PHAB_ID=D525860``
 
@@ -82,8 +82,8 @@ No Changes
 * scrooge-generator: Optimized generated Scala code for compile time and smaller bytecode.
   Companion objects for thrift enum traits are no longer case objects. ``PHAB_ID=D454297``
 
-20.4.0
-------
+20.4.0 (DO NOT USE)
+-------------------
 
 No Changes