[Az.Accounts] Remove duplicated code copied from Azure.Identity (#25733)

* use Azure.Identity

* update README.md

* update ChangeLog.md

* move TaskExtensions.cs to src/Accounts/Authentication/KeyStore

* Update ChangeLog.md

Polish change log

---------

Co-authored-by: Jin Lei <[email protected]>

Author: YanaXu

src/Accounts/Accounts/ChangeLog.md

@@ -19,6 +19,7 @@
 -->
 
 ## Upcoming Release
+* Used Azure.Identity and Azure.Core directly for client assertion [#22628].
 
 ## Version 3.0.3
 * Reduced the frequency of displaying sign-in announcement messages.