Add changelog for 3.3.4-RC2

changelogs/3.3.4-RC2.md

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+# Backported fixes
+
+- Fix a bundle of patmat issues [#21000](https://github.com/scala/scala3/pull/21000)
+- Fixed false positive unreachable local object [#21532](https://github.com/scala/scala3/pull/21532)
+- Reimplement support for type aliases in SAM types [#18317](https://github.com/scala/scala3/pull/18317)
+- Allow SAM types to contain match alias refinements [#20092](https://github.com/scala/scala3/pull/20092)
+- Allow SAM types to contain multiple refinements [#20172](https://github.com/scala/scala3/pull/20172)
+
+# Contributors
+
+Thank you to all the contributors who made this release possible 🎉
+
+According to `git shortlog -sn --no-merges 3.3.4-RC1..3.3.4-RC2` these are:
+
+```
+    11  Dale Wijnand
+     7  Wojciech Mazur
+     1  Guillaume Martres
+```