Twitter-oss: Prepare OSS libraries for release 21.8.0

Problem

We want to release the next versions of our Twitter OSS libraries 21.8.0

    util
    scrooge
    finagle
    twitter-server
    finatra

Solution

Prepare libraries for their next releases.

JIRA Issues: CSL-11162

Differential Revision: https://phabricator.twitter.biz/D723532

CHANGELOG.rst

@@ -7,13 +7,8 @@ Note that ``PHAB_ID=#`` and ``RB_ID=#`` correspond to associated messages in com
 Unreleased
 ----------
 
-21.8.0
-------
-
-No Changes
-
-21.7.0 (No Release)
--------------------
+21.8.0 (No 21.7.0 Release)
+--------------------------
 
 Admin Endpoint Versions
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