fix: exempt changelogs from CODEOWNERS (#15712)

Author: ncooke3

.github/CODEOWNERS

@@ -9,3 +9,4 @@ Interop/ @firebase/firebase-ios-sdk-admin
 scripts/ @firebase/firebase-ios-sdk-admin
 .github/ @firebase/firebase-ios-sdk-admin
 *.md @firebase/firebase-ios-sdk-admin
+CHANGELOG.md # Exempt from the `*.md` rule.