Add changelog for 3.4.3-RC1

Author: WojciechMazur

changelogs/3.4.3-RC1.md

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+# Highlights of the release
+
+- Revert "Approximate MatchTypes with lub of case bodies, if non-recursive" in 3.4.3 [#21268](https://github.com/scala/scala3/pull/21268)
+
+# Contributors
+
+Thank you to all the contributors who made this release possible 🎉
+
+According to `git shortlog -sn --no-merges 3.4.2..3.4.3` these are:
+
+```
+    6  Wojciech Mazur
+```