Add changelog for 3.4.0-RC3

Author: Kordyjan

changelogs/3.4.0-RC3.md

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+# Backported fixes
+
+- Sync language.scala with main and backport "Add tests for context bounds migration" [#19515] (https://github.com/lampepfl/dotty/pull/19515)
+- Handle default implicits to context parameters under -3.4-migration [#19512] (https://github.com/lampepfl/dotty/pull/19512)
+
+# Contributors
+
+Thank you to all the contributors who made this release possible 🎉
+
+According to `git shortlog -sn --no-merges 3.4.0-RC2..3.4.0-RC3` these are:
+
+```
+     4	Paweł Marks
+     1	Martin Odersky
+     1	Nicolas Stucki
+
+```