update changelog for 6.6.5 release (#4606)

Signed-off-by: Justin Kolberg <[email protected]>

Author: amdprophet

CHANGELOG.md

@@ -6,7 +6,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic
 Versioning](http://semver.org/spec/v2.0.0.html).
 
-## Unreleased
+## [6.6.5] - 2022-02-03
 
 ### Fixed
 - Fixed interactive wording for `sensu-backend upgrade`. The wording no longer