Rename "reference block" to "execution block"

general note for another discussion/PR: I wonder if the name "reference block" could be improved, I personally don't find it very intuitive: it's not clear what it _refers_ to.

I think "execution block" would do more justice to its role.

_Originally posted by @mmagician in https://github.com/0xMiden/protocol/pull/3737#discussion_r3861516291_