Add aria-modal attribute to Dialog (V2) element

[iansan5653]

Adds the aria-modal attribute to the container for the DialogV2 element. This is required because the dialog always instantiates a focus trap. The only way to interact with the underlying page is to close the dialog:

Relevant only on dialog and alertdialog containers, setting aria-modal="true" tells assistive technologies to let the user know the ability to interact with, or access other content on the page requires the modal dialog to be closed or otherwise lose focus. (MDN)

Adding aria-modal prevents screen readers from continuing past the dialog and starting to read the underlying content, which they will do without the attribute.

Closes #2814

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Latest commit: d8745e0

The changes in this PR will be included in the next version bump.

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@primer/react	Patch

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[iansan5653]

I considered adding a prop to allow users to override this, but since we don't allow them to override the focus zone or exit behavior, there is no valid scenario in which this dialog is not modal. And the role prop is restricted to dialog or alertdialog so there is no potential for conflict there.

[github-actions]

size-limit report 📦

Path	Size
dist/browser.esm.js	89.67 KB (-0.01% 🔽)
dist/browser.umd.js	90.29 KB (+0.01% 🔺)