Replace JDK 21 method usage to allow running/compiling on JDK 17

Author: beikov

hibernate-core/src/test/java/org/hibernate/orm/test/mapping/access/ElementCollectionInOneToManyTest.java

@@ -71,7 +71,7 @@ void hhh13721Test(SessionFactoryScope scope) {
 			assertEquals( "author", author.name );
 			assertEquals( 1, author.books.size() );
 
-			Book book = author.books.getFirst();
+			Book book = author.books.get( 0 );
 			assertEquals( "book", book.name );
 			assertEquals( 3, book.chapters.size() );