Change callback prop return type from `any` to `void`

This ensures that the component doesn't attempt to call the prop and use
the result.

src/sidebar/components/Excerpt.js

@@ -56,9 +56,9 @@ const noop = () => {};
  *   when it is collapsed.
  * @prop {number} [overflowThreshold] - An additional margin of pixels by which
  *   the content height can exceed `collapsedHeight` before it becomes collapsible.
- * @prop {(isCollapsible: boolean) => any} [onCollapsibleChanged] - Called when the content height
+ * @prop {(isCollapsible: boolean) => void} [onCollapsibleChanged] - Called when the content height
  *   exceeds or falls below `collapsedHeight + overflowThreshold`.
- * @prop {(collapsed: boolean) => any} [onToggleCollapsed] - When `inlineControls` is `false`, this
+ * @prop {(collapsed: boolean) => void} [onToggleCollapsed] - When `inlineControls` is `false`, this
  *   function is called when the user requests to expand the content by clicking a
  *   zone at the bottom of the container.
  * @prop {Object} [settings] - Used for theming.