update cas docs with attribute release for 2.0 protocol setting

[Jason Everling]

I never knew this was added to cas, I somehow stumbled upon it after looking over an updated default application.properties, it essentially adds the attribute release to the 2.0 protocol. I was still using a modded 2.0 view and the later started using the method described here, https://groups.google.com/a/apereo.org/d/msg/cas-user/kA8d_1Jdsvo/w4K4EXYQCAAJ , but you no longer have to do any of this as posted here, https://apereo.github.io/2017/06/23/cas-protocol-compatibility/ you just need to add the below to you cas.properties and viola, all those modifications are no longer needed. It is one less thing we need maintain during upgrades.

cas.view.cas2.v3ForwardCompatible=true

Maybe this can be added to the CAS Protocol page? I checked as much as I could and didn't see it mentioned anywhere.

https://apereo.github.io/cas/6.1.x/protocol/CAS-Protocol.html