Add changelog for 3.5.0-RC6

Author: WojciechMazur

changelogs/3.5.0-RC6.md

@@ -0,0 +1,13 @@
+# Backported fixes
+
+- Revert "Approximate MatchTypes with lub of case bodies, if non-recursive" in 3.5.0 [#21266](https://github.com/scala/scala3/pull/21266)
+
+# Contributors
+
+Thank you to all the contributors who made this release possible 🎉
+
+According to `git shortlog -sn --no-merges 3.5.0-RC5..3.5.0-RC6` these are:
+
+```
+     4  Wojciech Mazur
+```