Twitter-oss: Prepare OSS libraries for release 21.9.0

Problem

We want to release the next versions of our Twitter OSS libraries 21.9.0
 - util
 - scrooge
 - finagle
 - twitter-server
 - finatra

Solution

Prepare libraries for their next releases.

Differential Revision: https://phabricator.twitter.biz/D754337

CHANGELOG.rst

@@ -7,6 +7,9 @@ Note that ``PHAB_ID=#`` and ``RB_ID=#`` correspond to associated messages in com
 Unreleased
 ----------
 
+21.9.0
+------
+
 New Features
 ~~~~~~~~~~~~
 

README.md

@@ -1,6 +1,6 @@
 # Twitter Util
 
-[![Build Status](https://github.com/twitter/util/workflows/continuous%20integration/badge.svg?branch=develop)](https://github.com/twitter/util/actions?query=workflow%3A%22continuous+integration%22+branch%3Adevelop)
+[![Build Status](https://github.com/twitter/util/workflows/continuous%20integration/badge.svg?branch=release)](https://github.com/twitter/util/actions?query=workflow%3A%22continuous+integration%22+branch%3Arelease)
 [![Codecov](https://codecov.io/gh/twitter/util/branch/develop/graph/badge.svg)](https://codecov.io/gh/twitter/util)
 [![Project status](https://img.shields.io/badge/status-active-brightgreen.svg)](#status)
 [![Gitter](https://badges.gitter.im/twitter/finagle.svg)](https://gitter.im/twitter/finagle?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)
@@ -36,7 +36,7 @@ for more details about how to contribute.
 An example SBT dependency string for the `util-core` library would look like this:
 
 ```scala
-val utilCore = "com.twitter" %% "util-core" % "21.8.0"
+val utilCore = "com.twitter" %% "util-core" % "21.9.0"
 ```
 
 # Units

build.sbt

@@ -4,7 +4,7 @@ Global / onChangedBuildSource := ReloadOnSourceChanges
 Global / excludeLintKeys += scalacOptions // might be actually unused in util-doc module but not sure
 
 // All Twitter library releases are date versioned as YY.MM.patch
-val releaseVersion = "21.9.0-SNAPSHOT"
+val releaseVersion = "21.9.0"
 
 val slf4jVersion = "1.7.30"
 val jacksonVersion = "2.11.4"