Fix group block editor block class name

[talldan]

Description

Fixes #20140.

#19181 seems to have accidentally removed the use of the className prop in the group block's edit function. This was causing custom block styles and the Additional CSS Classes option not to function in the editor. This PR restores use of the className prop.

How has this been tested?

1. Add a class name (e.g. 'my-class') to a group block using the Advanced > Additional CSS Class(es) option
2. Inspect the group block using dev tools. The block should have the classname matching the one added above (e.g. 'wp-block-group my-class')

---

1. Register a block style by adding something like the following

register_block_style( 'core/group', [ 'name' => 'golden-border', 'label' => 'Golden Border', 'style_handle' => 'block-styles-stylesheet', ] );

2. Reload the editor and toggle the style on a group block
3. Inspect the group block using the dev tools. The block should have the class name 'wp-block-group is-style-golden-border'.

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.