Rework Spring Session Redis properties to make it clearer which properties apply in which situations

We have 6 properties at the moment:

• spring.session.redis.cleanup-cron
• spring.session.redis.configure-action
• spring.session.redis.flush-mode
• spring.session.redis.namespace
• spring.session.redis.repository-type
• spring.session.redis.save-mode

Of these, two are servlet-specific:

• spring.session.redis.cleanup-cron
• spring.session.redis.flush-mode

And two are specific to indexed repositories:

• spring.session.redis.cleanup-cron
• spring.session.redis.configure-action

We should try to rework the properties and how they're handled to make this more clear. For example, we could move the servlet-specific properties into a group of their own:

• spring.session.redis.servlet.cleanup-cron
• spring.session.redis.servlet.flush-mode

We could also fail if an indexed-specific property is set when using the default repository type. Additionally or alternatively we could move the indexed-specific properties into their own group. This would leave us with these 6 properties:

• spring.session.redis.servlet.indexed.cleanup-cron
• spring.session.redis.indexed.configure-action
• spring.session.redis.servlet.flush-mode
• spring.session.redis.namespace
• spring.session.redis.repository-type
• spring.session.redis.save-mode

Start by locating the Spring Session Redis property definitions and the code that handles repository type, servlet behavior, and indexed repositories. Review how the six existing properties are bound and validated. Done means the property structure and handling clearly communicate which settings apply to servlet and indexed repositories, including the behavior for incompatible settings.