Add changelog for 3.4.0-RC2

Author: Kordyjan

changelogs/3.4.0-RC2.md

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+# Backported fixes
+
+- Fix expandParam's use of argForParam/isArgPrefixOf. [#19412](https://github.com/lampepfl/dotty/pull/19412)
+- Remove ascriptionVarargsUnpacking as we never used it [#19399](https://github.com/lampepfl/dotty/pull/19399)
+- Make explicit arguments for context bounds an error from 3.5 [#19316](https://github.com/lampepfl/dotty/pull/19316)
+- Avoid generating given definitions that loop [#19282](https://github.com/lampepfl/dotty/pull/19282)
+- Turn given loop prevention on for -source future [#19392](https://github.com/lampepfl/dotty/pull/19392)
+- Fix algorithm to prevent recursive givens [#19411](https://github.com/lampepfl/dotty/pull/19411)
+
+# Contributors
+
+Thank you to all the contributors who made this release possible 🎉
+
+According to `git shortlog -sn --no-merges 3.4.0-RC1..3.4.0-RC2` these are:
+
+```
+    15	Martin Odersky
+     4	Nicolas Stucki
+     3	Paweł Marks
+     1	Dale Wijnand
+     1	Jan Chyb
+```