Add changelog for 3.3.1-RC5

Author: Kordyjan

changelogs/3.3.1-RC5.md

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+# Backported fixes
+
+- Heal stage inconsistent prefixes of type projections [#18239](https://github.com/lampepfl/dotty/pull/18239)
+- Fix regression #17245: Overloaded methods with ClassTags [#18286](http://github.com/lampepfl/dotty/pull/18286)
+- Disallow taking singleton types of packages again [#18232](http://github.com/lampepfl/dotty/pull/18232)
+- A slightly more conservative version of #14218 [#18352](http://github.com/lampepfl/dotty/pull/18352)
+- Record failures to adapt application arguments [#18269](http://github.com/lampepfl/dotty/pull/18269)
+- Fix regression in exhaustivity of HK types [#18303](http://github.com/lampepfl/dotty/pull/18303)
+
+# 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:
+
+```
+     5 Dale Wijnand
+     2 Martin Odersky
+     2 Paweł Marks
+     1 Jan Chyb
+     1 Nicolas Stucki
+```