Components: Remove "experimental" designation for CustomSelectControlV2

[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.

[fullofcaffeine]

Note to self: follow-up this (or work in parallel) with a PR to change the default exported implementation to the new legacy adapter from V2. I started playing with it in this commit: 752a07f, and found some API problems with the new adapter.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: fullofcaffeine <[email protected]>
Co-authored-by: mirka <[email protected]>
Co-authored-by: tyxla <[email protected]>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[tyxla]

Should we also remove the experimental callout from the component README?

gutenberg/packages/components/src/custom-select-control-v2/README.md

Line 4 in ab708fd

This feature is still experimental. “Experimental” means this is an early implementation subject to drastic and breaking changes.

Otherwise, looks good to me 👍