Add layout classes to legacy Group block inner container.

[isabel_brison]

With classic themes, Group block markup has an inner wrapper around its children. This means that if layout support is enabled via appearance-tools as proposed in #60118, block spacing won't work correctly because the layout class is still applied to the outer wrapper.

This has been fixed in Gutenberg with ​https://github.com/WordPress/gutenberg/pull/56130. This ticket tracks syncing that PR to core.

Trac ticket: https://core.trac.wordpress.org/ticket/60130

Syncs the changes from ​https://github.com/WordPress/gutenberg/pull/56130 to core. Editor-side changes will be added with the npm package updates as usual; this PR will work without them but classname changes will only be visible in the front end.

To test:

1. Activate a classic theme;
2. In the editor, add a Group block with some children;
3. Save and publish;
4. Check that classnames is-layout-constrained wp-block-group-is-layout-constrained are added to the inner container on the front end.
5. Run the layout unit tests.

Thanks for the feedback folks! All points have been addressed now.

[isabel_brison]

In 57246:

Editor: add layout classes to legacy Group inner container.

Moves generated layout classes into the Group block inner container in classic themes, so that block gap support can work correctly.

Props flixos90, mukesh27.
Fixes #60130.

Committed in r57246.