Review ServerProperties and document keys that only work in certain environments

[philwebb]

Currently ServerProperties contains quite a mix of things. We'll need to refactor it to get cleaner reactive/servlet concerns.

[philwebb]

We can probably also unify some of DefaultServletWebServerFactoryCustomizer and DefaultReactiveWebServerFactoryCustomizer

[wilkinsona]

The current arrangement requires us to allow org.springframework.boot.web.servlet.server to be imported into org.springframework.boot.autoconfigure.web as org.springframework.boot.web.servlet.server.Jsp is used by ServerProperties.

As part of fixing this we should update Checkstyle's import controls to lock that back down.