Document function drops & changes in migration

Author: MartijnCuppens

site/content/docs/4.3/migration.md

@@ -33,6 +33,8 @@ Changes to our source Sass files and compiled CSS.
 - **Todo:** Rearrange forms source files (under `scss/forms/`)
 - **Todo:** Rearrange grid source files (under `scss/grid/`)
 - Removed print styles and `$enable-print-styles` variable. Print display classes, however, have remained intact. [See #28339](https://github.com/twbs/bootstrap/pull/28339).
+- Drop `color()`, `theme-color()` & `gray()` functions  in favor of variables. [See #29083](https://github.com/twbs/bootstrap/pull/29083)
+- The `theme-color-level()` now accepts any color you want instead of only `$theme-color` colors. [See #29083](https://github.com/twbs/bootstrap/pull/29083)
 
 ## JavaScript