Components: Assess stabilization of Spacer

[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.