Social Link: Fix server-side rendering for legacy format (#20074)

Fixes a regression introduced in #19887.

While changing the logic in `register_block_core_social_link` to consume
the block type's `block.json` and switch to the `$service` attribute
name, I broke the handling of the legacy format for social links.

Old format: <!-- wp:social-link-wordpress {"url":"foo"} /-->
New format: <!-- wp:social-link {"service":"wordpress","url":"foo"} /-->

This fixes the handling of legacy social links by providing the adequate
attribute default for each `$service`.

packages/block-library/src/social-link/index.php

@@ -81,6 +81,18 @@ function register_block_core_social_link() {
 			array_merge(
 				$metadata,
 				array(
+					'attributes'      => array(
+						'url'     => array(
+							'type' => 'string',
+						),
+						'service' => array(
+							'type'    => 'string',
+							'default' => $site,
+						),
+						'label'   => array(
+							'type' => 'string',
+						),
+					),
 					'render_callback' => 'render_core_social_link',
 				)
 			)