Twitter-oss: Prepare OSS libraries for release 21.6.0

Problem

We want to release the next versions of our Twitter OSS libraries 21.6.0
 - util
 - scrooge
 - finagle
 - twitter-server
 - finatra

Solution

Prepare libraries for their next releases.

Differential Revision: https://phabricator.twitter.biz/D692183

CHANGELOG.rst

@@ -7,6 +7,9 @@ Note that ``RB_ID=#`` and ``PHAB_ID=#`` correspond to associated message in comm
 Unreleased
 ----------
 
+21.6.0
+------
+
 Changed
 ~~~~~~~
 

README.md

@@ -1,7 +1,7 @@
 # Finatra
 
-[![Build Status](https://github.com/twitter/finatra/workflows/continuous%20integration/badge.svg?branch=develop)](https://github.com/twitter/finatra/actions?query=workflow%3A%22continuous+integration%22+branch%3Adevelop)
-[![Test Coverage](https://codecov.io/github/twitter/finatra/coverage.svg?branch=develop)](https://codecov.io/github/twitter/finatra?branch=develop)
+[![Build Status](https://github.com/twitter/finatra/workflows/continuous%20integration/badge.svg?branch=release)](https://github.com/twitter/finatra/actions?query=workflow%3A%22continuous+integration%22+branch%3Arelease)
+[![Test Coverage](https://codecov.io/github/twitter/finatra/coverage.svg?branch=release)](https://codecov.io/github/twitter/finatra?branch=release)
 [![Project status](https://img.shields.io/badge/status-active-brightgreen.svg)](#status)
 [![Maven Central](https://maven-badges.herokuapp.com/maven-central/com.twitter/finatra-http-server_2.12/badge.svg)][maven-central]
 [![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/twitter/finatra)

build.sbt

@@ -6,7 +6,7 @@ Global / excludeLintKeys += scalacOptions
 Global / concurrentRestrictions += Tags.limit(Tags.Test, 1)
 
 // All Twitter library releases are date versioned as YY.MM.patch
-val releaseVersion = "21.6.0-SNAPSHOT"
+val releaseVersion = "21.6.0"
 
 lazy val buildSettings = Seq(
   version := releaseVersion,

project/plugins.sbt

@@ -3,7 +3,7 @@ resolvers ++= Seq(
   Resolver.sonatypeRepo("snapshots")
 )
 
-val releaseVersion = "21.6.0-SNAPSHOT"
+val releaseVersion = "21.6.0"
 
 addSbtPlugin("com.twitter" % "scrooge-sbt-plugin" % releaseVersion)