ConfirmDialog: Add __next40pxDefaultSize to buttons

[t-hamano]

What?

This PR adds a __next40pxDefaultSize prop to make the two buttons inside the ConfirmDialog component 40px tall. As a result, the issue reported in #58374 will also be closed.

Before	After

Why?

Support the consistent component effort in #46734.

Also, for example, in the Site Editor's renameing modal, the button height is already 40px. By applying this PR, the height of the buttons in the rename/delete modal dialog will be unified.

Testing Instructions

• Launch Storybook and access the following URL to check the button height: http://localhost:50240/?path=/story/components-experimental-confirmdialog--default
• Access the Site Editor and open a modal dialog where the ConfirmDialog component is used. For example, click the Delete button in the dropdown on the template page.
• Make sure the height of the button is 40px.

[github-actions]

Flaky tests detected in b43000e.
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/7706495986
📝 Reported issues:

• [Flaky Test] directives inside islands should not be hydrated twice #58332 in /test/e2e/specs/interactivity/tovdom-islands.spec.ts

[mirka]

Nice, thank you! 🚀