Add changelog for 3.3.1-RC6

[Cherry-picked 340303f]

Author: Kordyjan

changelogs/3.3.1-RC6.md

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+# Backported fixes
+
+- Refine `infoDependsOnPrefix` [#18204](https://github.com/lampepfl/dotty/pull/18204)
+- FDo not compute `protoFormal` if `param.tpt` is empty [#18288](http://github.com/lampepfl/dotty/pull/18288)
+- Revert "Normalize match type usage during implicit lookup" [#18440](http://github.com/lampepfl/dotty/pull/18440)
+
+# Contributors
+
+Thank you to all the contributors who made this release possible 🎉
+
+According to `git shortlog -sn --no-merges 3.3.1-RC4..3.3.1-RC5` these are:
+
+```
+     3	Paweł Marks
+     2	Martin Odersky
+     1	Nicolas Stucki
+```