Update release notes to mention WinUI dependency change (#5656)

See #4970
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Author: florelis

doc/ReleaseNotes.md

@@ -1,5 +1,6 @@
 ## New in v1.12
-MCP server available; run `winget mcp` for assistance on configuring your client.
+* MCP server available; run `winget mcp` for assistance on configuring your client.
+* App Installer now uses WinUI 3. The package dependency on WinUI 2 has been replaced by a dependency on the Windows App Runtime 1.7.
 
 ## Bug Fixes
 * Manifest validation no longer fails using `UTF-8 BOM` encoding when the schema header is on the first line