There is also a related site setting default topics automatic unpin that seems to be for the same thing; the definitions are ambiguous. I don’t know why there are two settings that seem identical but maybe someone else does.
I haven’t tried to adjust this behavior but one or both of these settings should do it.
Nobody who uses your forum prefers this though. If you make it so each time they open a topic they start at the beginning instead of the last message read, it will be by far the #1 complaint your users have.
If you want to make it clear how users can go back to the top of the topic you can add something like
One of my discourse sites is used as a wiki, I’d like to be able to have posts load from the top as well.
It makes sense to start at the top for wiki posts.