Comment pagination numbers: add background color

[carolinan]

What?

Updates the block supports to include background color.
Related: #43245

Why?

To match the settings of the comments pagination next and previous inner blocks.

How?

By updating the block support and documentation.

Testing Instructions

1. First make sure that your test installation has some comments, and that comment pagination is enabled in the WordPress admin area under Settings, Discussion, "Break comments into pages"
2. Create or open a post or page template in the site editor and add a comments block.
3. In the comments block, locate the nested comment pagination numbers block.
4. Confirm if the block has background color and background gradient settings in the block sidebar. Change the color.
5. Save and view the front, confirm if the color is applied on the front.

Add the example code below to theme.json under styles.blocks.
Confirm that the color displays correctly in the editor and front, and that the colors can be changed in the block settings sidebar.

"core/comments-pagination-numbers": {
	"color": {
			"background": "green"
	}
}

Continue to test the block's background and gradient color in the global styles sidebar in the site editor.

[aristath]

LGTM 👍