add changelog entry for object pruning

CHANGELOG.md

@@ -39,6 +39,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Metadata: RelMe provider
 - OAuth: added support for refresh tokens
 - Emoji packs and emoji pack manager
+- Object pruning (`mix pleroma.database prune_objects`)
 
 ### Changed
 - **Breaking:** Configuration: move from Pleroma.Mailer to Pleroma.Emails.Mailer