Document the new Active Support cache format

[casperisfine]

Followup: #42025

Hopefully this is clear enough?

I also think we should immediately deprecate the 6.1 format as it constrain the Entry internal state preventing various cleanup and optimizations. Not sure what the window should be though, Rails 7.2 ?

cc @rafaelfranca @fxn ?

[fxn]

@casperisfine In principle 7.2 could be a good compromise, I think.

So, we could issue a deprecation warning if 6.1 format is configured in a 7.1 application, and remove support in 7.2. I guess with such a wide time span caches are unlikely to have old entries, which is important because that would become an error condition. However, unless you scan the cache I guess you cannot have certainty in a generic situation?

[casperisfine]

Ok, sounds good. I'll add a deprecation once 7.0 is out.