docs: update CHANGELOG for v20.1 (#31511)

Author: andrewseguin

CHANGELOG.md

@@ -1,5 +1,11 @@
 <a name="20.1.0"></a>
 # 20.1.0 "metal-brownie" (2025-07-09)
+There have been some internal mapping changes in Material 2 theme config maps generated by `mat.define-light-theme` and `mat.define-dark-theme`.
+
+If your app was directly modifying these internal properties to alter styles, such as setting new color values used in the foreground and background palette maps, then your changes may no longer be applied to the components. The supported path for changing theme values is through each component’s overrides API.
+
+The Material 2 component styles have undergone minor adjustments to enhance consistency across components and better align with the latest Material 2 specification. These changes result in a closer adherence to system-level colors such as `surface`, `background`, `disabled`, and `surface` variants. In general, these changes are slight hue differences for grey colors that aren’t very noticeable.
+
 ### cdk
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@@ -6213,4 +6219,4 @@ _Breaking changes:_
 
 # Changes Prior to 12.0.0
 
-To view changes that occurred prior to 12.0.0, see [CHANGELOG_ARCHIVE.md](https://github.com/angular/components/blob/main/CHANGELOG_ARCHIVE.md).
+To view changes that occurred prior to 12.0.0, see [CHANGELOG_ARCHIVE.md](https://github.com/angular/components/blob/main/CHANGELOG_ARCHIVE.md).