Update Gemfile fast_xor platform to :ruby

[Falco]

@eregon

Can Discourse run on Puma instead, any configuration needed for that?

In development a simple rails server will use puma.

@eregon

I wonder, for context, why is JS used to render Markdown?
To make sure it’s exactly the same result as when run in the client browser?

Yes. Now that wasm exists it may be a way out, but it would be a very big change to keep it as extensible by plugins as it is today.