Remove usage of .components-item-group CSS selector from edit-site components

[afercia]

Description

Splitting this out from #67321 (comment)

Cc @t-hamano @jameskoster

CSS selectors that come from base components of the @wordpress/components package should not be used in other packages.

This is the only place in all of Gutenberg where the .components-item-group selector is used.
It should be replaced with a more specific and 'semantically' correct CSS class. The class should be applied to all the <ItemGroup> components used in the edit site navigation screen and all its sub-screens,

Introduced in #50767

gutenberg/packages/edit-site/src/components/sidebar-navigation-screen/style.scss

Lines 21 to 24 in b54d1fe

	.components-item-group {
	margin-left: -$grid-unit-20;
	margin-right: -$grid-unit-20;
	}

Step-by-step reproduction instructions

• Search in the codebase for usages of the .components-item-group CSS selector.
• Observe there's only one usage in a component of the edit-site package.

Screenshots, screen recording, code snippet

No response

Environment info

No response

Please confirm that you have searched existing issues in the repo.

• Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

• Yes

Please confirm which theme type you used for testing.

• Block
• Classic
• Hybrid (e.g. classic with theme.json)
• Not sure