SpringBoot EnvironmentPostProcessors aren't instantiated with correct ClassLoader

Similar to the issue raised in https://github.com/spring-projects/spring-boot/issues/26126, when using Spring Boot 2.5.0 org.springframework.boot.env.ReflectionEnvironmentPostProcessorsFactory will not use the class loader from the SpringApplication's ResourceLoader when instantiating EnvironmentPostProcessors.

It appears to me the classloader would need to be passed to the instantiator here https://github.com/spring-projects/spring-boot/blob/07c9418b85da40d1b981314100629d98326c516b/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/env/ReflectionEnvironmentPostProcessorsFactory.java#L63. That way https://github.com/spring-projects/spring-boot/blob/07c9418b85da40d1b981314100629d98326c516b/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/util/Instantiator.java#L98 could use the correct classloader rather than being passed "null". ReflectionEnvironmentPostProcessorsFactory would need to be made aware of the SpringApplication's ClassLoader as well.

Comment From: philwebb

There are two parts to this bug. The first is that we're not passing a classloader to the Instantiator, the second is that we want to use ResourceLoader.getClassloader().

I'll use this issue to fix the first problem in 2.4.x. The second part we'll fix in 2.5.x under #27071