fix: filterItems prop should be optional in FilterableMultiSelectProps (#18581)

Co-authored-by: Gururaj J <89023023+Gururajj77@users.noreply.github.com>

Author: martinscooper

packages/react/src/components/MultiSelect/FilterableMultiSelect.tsx

@@ -159,7 +159,7 @@ export interface FilterableMultiSelectProps<ItemType>
   /**
    * Default sorter is assigned if not provided.
    */
-  filterItems(
+  filterItems?(
     items: readonly ItemType[],
     extra: {
       inputValue: string | null;