Deprecate Authentication#setAuthenticated

[jzheaux]

Authentication#setAuthenticated is not used by Spring Security other than inside of the constructors of Authentication implementations. Further, it's not a great look for an otherwise immutable authentication state to have it's validity be publicly mutable.

For this ticket:

• Mark Authentication#setAuthenticated as @Deprecated.
• Add to the JavaDoc, indicating what folks should do (implement isAuthenticated instead)
• Add to the migration doc instructions to stop using Authentication#setAuthenticated

Existing implementations will need to still call it in order to remain passive, so they should not change in this ticket. That will be in when setAuthenticated is removed.