Modal - create variation using `dialog` element

Is this a new component or an extension of an existing one?
The Modal creates a ModalBox, which will have a variation in Core built using the <dialog> element.

Describe the feature
<dialog> provides functionality built into browsers that handles placing the modal on the top layer, handling keyboard focus, and providing a ::backdrop pseudoelement. Users should be able to opt in to this variant, which then probably does not need the separate Backdrop component, may not need to be placed in the DOM where it is now, and focus will be handled by the browser. A11y should be reviewed given that the dialog implicitly provides appropriate aria labeling.

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/dialog

This issue is dependent on completion in core in https://github.com/patternfly/patternfly/issues/7296

首先检查现有 React Modal 和 ModalBox 的行为，然后阅读链接的 core issue 7296 以及 MDN 的 dialog 文档。检查当前如何使用 Backdrop 组件和焦点处理。完成的标准是：一个基于 dialog、通过 opt-in 启用的变体能够正常工作并具备适当的无障碍行为，同时其与 Backdrop 的交互以及在 DOM 中的位置问题已得到解决。