updating change log

Author: jeffrymsft

changelog.txt

@@ -14,6 +14,7 @@ Version 21.0.0
 - [PATCH] Fix multiple prompts issue in cross cloud request (#2599)
 - [PATCH] Corrected error handling in cross cloud scenario (#2602)
 - [MINOR] Native auth: Add claimsRequest also to getAccessToken and signIn after signUp/SSPR flows (#2622)
+- [MINOR] Upgrading AGP Version to 8.1.0 (#2587)
 
 Version 20.1.1
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