changelog: update for 3.21.8

Author: mrnugget

CHANGELOG.md

@@ -17,6 +17,10 @@ All notable changes to `src-cli` are documented in this file.
 
 ### Fixed
 
+## 3.21.8
+
+### Fixed
+
 - `src campaign [apply|preview]` could fail to parse the produced diff in a repository when `git` was configured to use a custom `diff` program. The fix is to ignore any local `git` configuration when running `git` commands. [#373](https://github.com/sourcegraph/src-cli/pull/373)
 
 ### Removed