changelogs: Mark version 20.4.0 and 20.8.0 do not use
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

twitter/finatra,twitter/finatra,twitter/finatra