Add changelog for 3.2.1-RC3

Author: Kordyjan

changelogs/3.2.1-RC3.md

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+# Backported fixes
+
+- Fix tuple casting [#16113](https://github.com/lampepfl/dotty/pull/16113)
+- Be even more careful when combining argument and info in `computeAsSeenFrom` [#16070](https://github.com/lampepfl/dotty/pull/16070)
+- Be still more careful when computing denotations of class parameters [#16112](https://github.com/lampepfl/dotty/pull/16112)
+
+# Contributors
+
+Thank you to all the contributors who made this release possible 🎉
+
+According to `git shortlog -sn --no-merges 3.2.1-RC2..3.2.1-RC3` these are:
+
+```
+     4  Martin Odersky
+     2  Paweł Marks
+```