Update changelog

Author: imjoehaines

CHANGELOG.md

@@ -1,7 +1,9 @@
 Changelog
 =========
 
-## TBD
+## v6.26.4 (25 March 2024)
+
+### Fixes
 
 * Fix Unicode encoding issues when using `Exception#detailed_message` (Ruby 3.2+)
   | [#817](https://github.com/bugsnag/bugsnag-ruby/pull/817)
@@ -11,6 +13,8 @@ Changelog
 
 ## v6.26.3 (24 January 2024)
 
+### Fixes
+
 * Handle mailto links in `Cleaner#clean_url`
   | [#813](https://github.com/bugsnag/bugsnag-ruby/pull/813)