Update CHANGELOG.md to document Peek API support.

PiperOrigin-RevId: 963649129

Author: Mobile Ads Developer Relations

ChangeLog.txt

@@ -5,6 +5,7 @@ Version Next
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 - Added support for forwarding request extras to Android ad request builders when using the Next-Gen SDK.
+- Added PeekAdResponseInfo() support for Preloading APIs to inspect first ad object in cache without removing it.
 
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 Version 11.3.0