Remove SecurityContextPersistenceFilter in Favor of Explicit Saves

[rwinch]

We should remove SecurityContextPersistenceFilter in favor of explicit saves to the SecurityContextRepository. This will provide lots of benefits:

• There will be no confusion when the SecurityContext should be saved
• Different types of authentication can save (or not save) the SecurityContext differently
• This would align with how WebFlux works

We would add a new Filter that only reads the SecurityContext and sets it on SecurityContextHolder. We should also consider providing a simplified API that doesn't involve needing to update the HttpRequestResponseHolder.