Editor: Finalize the theme_json_get_style_nodes hook name.

This consolidates the name of a filter introduced earlier in the 6.1 cycle, renaming it from `get_style_nodes` to `theme_json_get_style_nodes` as to be more coherent with the other hooks introduced in 6.1 as well.

This commit backports the original PR from Gutenberg repository:
* [https://github.com/WordPress/gutenberg/pull/44189 #44189 Use prefix 'theme_json_' in hooks related to theme.json]

Reference: [https://github.com/WordPress/wordpress-develop/pull/3247 #3247 Add hooks to filter theme.json data].

Follow-up to [54118].

Props oandregal, scruffian, bernhard-reiter.
See #56467.

git-svn-id: https://develop.svn.wordpress.org/trunk@54183 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-theme-json.php

@@ -1873,7 +1873,7 @@ class WP_Theme_JSON {
 		 *
 		 * @param array $nodes Style nodes with metadata.
 		 */
-		return apply_filters( 'get_style_nodes', $nodes );
+		return apply_filters( 'theme_json_get_style_nodes', $nodes );
 	}
 
 	/**