Remove spring.session.store-type in favor of a defined order for which implementation will win when multiple are available

[wilkinsona]

Looking at #27738 caused us to realize that the auto-configuration for Spring Session is more complex than it needs to be. To simplify it, we'd like to remove spring.session.store-type. In the unlikely event that multiple session repository implementations are available we should define and document which will be used, just as we do for JDBC connection pools today. If for some reason a user needs multiple session repository implementations on the classpath and our defined ordering doesn't meet their needs, they can define their own SessionRepository bean and cause the auto-configuration to back off.

[wilkinsona]

We've decided to reorder things a bit. We'd like the order to be:

1. Redis
2. JDBC
3. Hazelcast
4. Mongo