[EuiDelayRender] Migrate from class to function component

[weronikaolejniczak]

EuiDelayRender delays rendering its children until a specified time has elapsed. Used to prevent flash of loading content.

• File: components/delay_render/delay_render.tsx
• Class size: 52 lines
• State: toggle
• Lifecycle: componentDidMount, shouldComponentUpdate, componentDidUpdate, componentWillUnmount

Migration notes:

Small but uses shouldComponentUpdate for render gating. The shouldComponentUpdate pattern maps to React.memo or conditional rendering in the function body. Timer management maps to useEffect.

Acceptance criteria:

• Convert to function component with hooks
• Replace shouldComponentUpdate with React.memo or conditional render
• Maintain delay timing behavior
• All existing tests pass