Restore changelog entry removed in #23017. NFC

Author: kleisauke

ChangeLog.md

@@ -24,6 +24,7 @@ See docs/process.md for more on how version tagging works.
 3.1.73 - 11/28/24
 -----------------
 - libunwind was updated to LLVM 19.1.4. (#22394)
+- mimalloc was updated to 2.1.7. (#21548)
 
 3.1.72 - 11/19/24
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