Enable shadow support for cover block

On previous attempts to do this, the cover block was displaying
inconsistencies between how it was rendered in the editor vs the
frontend; that doesn't seem to be the case anymore, so we should be
enable shadow support without any modifications to the block itself.

I'd still appreciate some eyes on this, because I don't know what
exactly has changed since the first time I attempted this.

Tracking issue: #57103

docs/reference-guides/core-blocks.md

@@ -245,7 +245,7 @@ Add an image or video with a text overlay. ([Source](https://github.com/WordPres
 
 -	**Name:** core/cover
 -	**Category:** media
--	**Supports:** align, anchor, color (heading, text, ~~background~~, ~~enableContrastChecker~~), dimensions (aspectRatio), interactivity (clientNavigation), layout (~~allowJustification~~), spacing (blockGap, margin, padding), typography (fontSize, lineHeight), ~~html~~
+-	**Supports:** align, anchor, color (heading, text, ~~background~~, ~~enableContrastChecker~~), dimensions (aspectRatio), interactivity (clientNavigation), layout (~~allowJustification~~), shadow, spacing (blockGap, margin, padding), typography (fontSize, lineHeight), ~~html~~
 -	**Attributes:** allowedBlocks, alt, backgroundType, contentPosition, customGradient, customOverlayColor, dimRatio, focalPoint, gradient, hasParallax, id, isDark, isRepeated, isUserOverlayColor, minHeight, minHeightUnit, overlayColor, tagName, templateLock, url, useFeaturedImage
 
 ## Details

packages/block-library/src/cover/block.json

@@ -85,6 +85,7 @@
 		"anchor": true,
 		"align": true,
 		"html": false,
+		"shadow": true,
 		"spacing": {
 			"padding": true,
 			"margin": [ "top", "bottom" ],