Fix: the same set of related products is always displayed

get_related_products_query in the WC_Product_Data_Store_CPT class is
querying the suitable products with a "limit" clause, this causes
that products down in the table are never picked for display.
This commit adds a ORDER BY RAND() to the query so that all products
have a chance to get picked.

includes/data-stores/class-wc-product-data-store-cpt.php

@@ -1315,6 +1315,8 @@ class WC_Product_Data_Store_CPT extends WC_Data_Store_WP implements WC_Object_Da
 				AND p.post_type = 'product'
 
 			",
+			'orderby' => '
+				ORDER BY RAND()',
 			'limits' => '
 				LIMIT ' . absint( $limit ) . '
 			',