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

@@ -41,8 +41,8 @@ Breaking API Changes
 * Check SecurityManager permissions in the `ContentHandler` to ensure that contention
   snapshotting is allowed. ``PHAB_ID=D531873``
 
-20.8.0
-------
+20.8.0 (DO NOT USE)
+-------------------
 
 No Changes
 
@@ -82,8 +82,8 @@ Runtime Behavior Changes
 
 No Changes
 
-20.4.0
-------
+20.4.0 (DO NOT USE)
+-------------------
 
 No Changes