Upgrade to Spring Session 4.0.1

[snicoll]

Upgrade to Spring Session 4.0.1.

[mrksph]

Spring Session 4.0 reference doc is returning 404.

https://docs.spring.io/spring-session/reference/4.0/index.html

[philwebb]

Thanks @mrksph, I see you've raised spring-projects/spring-session#3511. We'll have to wait for a response on that issue.