Remove unreleased for 7.8 in changelog.

CHANGELOG

@@ -4,7 +4,7 @@ v 7.9.0 (unreleased)
   - Improve UI for commits, issues and merge request lists
   - Fix commit comments on first line of diff not rendering in Merge Request Discussion view.
 
-v 7.8.0 (unreleased)
+v 7.8.0
   - Fix access control and protection against XSS for note attachments and other uploads.
   - Replace highlight.js with rouge-fork rugments (Stefan Tatschner)
   - Make project search case insensitive (Hannes Rosenögger)