Update migration guide

Author: jzempel

docs/migration.md

@@ -173,6 +173,11 @@ consider additional positioning prop support on a case-by-case basis.
 - The default `theme` object has removed values for `colors.background` and
   `colors.foreground`. Use the `'background.default'` and `'foreground.default'`
   variables together with the v9 `getColor` utility instead.
+- The default theming `palette` has been redesigned and expanded to support
+  light/dark modes. A temporary, deprecated `getColorV8` utility has been added.
+  Use the temporary utility to apply legacy version 8 colors to custom
+  (non-Garden) components they can be upgraded to use the enhanced version 9
+  `getColor` utility.
 - Utility function `getDocument` has been removed. Use `useDocument` instead.
 - Utility function `isRtl` has been removed. Use `props.theme.rtl` instead.
 - The following exports have changed: