Use Jetpack_RelatedPosts::get_options() to get options for ser… (#14386)

This ensures that options are normalized and that they can be filtered via jetpack_relatedposts_filter_options.

modules/related-posts/jetpack-related-posts.php

@@ -194,7 +194,7 @@ public function filter_add_target_to_dom( $content ) {
 	 * @return string Rendered related posts HTML.
 	 */
 	public function get_server_rendered_html() {
-		$rp_settings       = Jetpack_Options::get_option( 'relatedposts', array() );
+		$rp_settings       = $this->get_options();
 		$block_rp_settings = array(
 			'displayThumbnails' => $rp_settings['show_thumbnails'],
 			'showHeadline'      => $rp_settings['show_headline'],