Modal: make everything outside inert

[stokesman]

What?

Bug fix/enhancement to Modal component.

Why?

I think inert is what we'd have used from the get go had it been well supported then.
To fix #41503
This should also help with other obscure problems like one that can be triggered by undo/redo keyboard shortcuts while a Modal is open.

a textbox underneath a modal will probably be focussed by the browser
—#18755 (comment)

How?

Updates use of aria-hidden to inert.

Testing Instructions

1. Open a post or page.
2. Launch a Modal (If you're testing a fresh instance and the Welcome guide popped up).
3. Click some part of your browser chrome
4. Use keyboard tab to return focus back to the Modal.

[github-actions]

Flaky tests detected in 97232f0.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/7579442555
📝 Reported issues:

• [Flaky Test] the event listener is removed when the element is removed #57983 in /test/e2e/specs/interactivity/directive-on-document.spec.ts