springboot Mark new public API added to spring-boot-loader in 2.3 with @since tags

While looking at https://github.com/spring-projects/spring-boot/issues/21575, I noticed that the new isExploded() method isn't documented as being @since 2.3.0 and I think it should be. We should also check for any other methods changed or added around the same time that need a similar update.