Add changelog for 3.6.4-RC2

Author: WojciechMazur

changelogs/3.6.4-RC2.md

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+# Reverted changes
+
+- Revert "Drop phase.isTyper use in isLegalPrefix/asf" from Scala 3.6.4 [#22653](https://github.com/lampepfl/dotty/pull/22653)
+
+# Contributors
+
+Thank you to all the contributors who made this release possible 🎉
+
+According to `git shortlog -sn --no-merges 3.6.4-RC1..3.6.4-RC2` these are:
+
+```
+     5  Wojciech Mazur
+```