refactor(toast): deprecate cssClass on ToastButton (#27959)

Author: liamdebeasi

core/src/components/toast/toast-interface.ts

@@ -25,11 +25,17 @@ export interface ToastOptions {
 
 export type ToastLayout = 'baseline' | 'stacked';
 
+// TODO FW-4923 remove cssClass property
+
 export interface ToastButton {
   text?: string;
   icon?: string;
   side?: 'start' | 'end';
   role?: 'cancel' | string;
+
+  /**
+   * @deprecated Use the toast button's CSS Shadow Parts instead.
+   */
   cssClass?: string | string[];
   htmlAttributes?: { [key: string]: any };
   handler?: () => boolean | void | Promise<boolean | void>;