Add changelog for 3.5.0-RC7

Author: WojciechMazur

changelogs/3.5.0-RC7.md

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+# Backported fixes
+
+- Backport "Fix healAmbiguous to compareAlternatives with disambiguate = true" to 3.5.0 [#21344](https://github.com/scala/scala3/pull/21344)
+
+# Contributors
+
+Thank you to all the contributors who made this release possible 🎉
+
+According to `git shortlog -sn --no-merges 3.5.0-RC6..3.5.0-RC7` these are:
+
+```
+     5  Martin Odersky
+     4  Wojciech Mazur
+     2  Eugene Flesselle
+```