[EuiAccordion] Migrate from class to function component

`EuiAccordion` is a collapsible content container with open/close toggle. Commonly used for progressive disclosure UI patterns.

- **File:** `components/accordion/accordion.tsx`
- **Class size:** 123 lines
- **State:** `isOpen`
- **Lifecycle:** None

**Migration notes:**

Simple — single boolean state, no lifecycle methods. The component is wrapped with `withEuiTheme` HOC which may also be replaceable with `useEuiTheme` hook.

**Acceptance criteria:**

- [ ] Convert to function component with `useState`
- [ ] Replace `withEuiTheme` HOC with `useEuiTheme` hook if applicable
- [ ] Maintain controlled/uncontrolled open state behavior
- [ ] All existing tests pass