Reintroduce previous EnvironmentPostProcessor in deprecated form

[philwebb]

We've had early feedback from the M3 release that the repackaging of EnvironmentPostProcessor causes some upgrade pain, especially for library authors who might have otherwise been able to get away with supporting Spring Boot 3.5 and 4.0 in the same release.

It might be possible for us to reintroduce the older class in a deprecated form if we're happy to live with the package tangle for a while.

[philwebb]

Here's a sample that shows the problem: https://github.com/philwebb/gh-47272-sample
Here's one possible fix: https://github.com/philwebb/spring-boot/tree/gh-47272