Components: Assess stabilization of ItemGroup

[fullofcaffeine]

Issue: #59418.

What?

This is part of a larger effort to remove __experimental prefix from all "experimental" components, effectively promoting them to regular stable components. See the related issue for more context.

Why?

The strategy of prefixing exports with __experimental has become deprecated after the introduction of private APIs.

How?

1. Export it from components without the __experimental prefix;
2. Keep the old __experimental export for backwards compatibility;
3. Change all imports of the old __experimental in GB and components to the one without the prefix (including in storybook stories). Also, update the docs to refer to the new unprefixed component;
4. Add the component storybook id (get it from the storybook URL) to the PREVIOUSLY_EXPERIMENTAL_COMPONENTS const array in manager-head.html so that old experimental story paths are redirected to the new one;
5. Add a changelog for the change.

[github-actions]

Flaky tests detected in 6692276.
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/8825151404
📝 Reported issues:

• [Flaky Test] Redirection after template creation #60678 in /test/e2e/specs/site-editor/site-editor-url-navigation.spec.js

[mirka]

We should look into how we manage height #63125 (comment)

[ciampo]

As per #63125 (comment), consensus was reached on:

• moving to a default size of 40px
• potentially deprecating the size prop (after carefully considering if we'll ever need a compact size?)
• treating the 40px height as a minimum height, meaning that children could push <Item /> to be taller. In that case, we'd need to be careful to balance min-height, text line-height and padding to make sure that multiple line text looks good, while also allowing non-text children to display with the right distance from the item borders (padding)