DEV: Refactor rp_id and rp_name (#23339)

They're both constant per-instance values, there is no need to store them
in the session. This also makes the code a bit more readable by moving
the `session_challenge_key` method up to the `DiscourseWebauthn` module.

app/models/concerns/second_factor_manager.rb

@@ -167,7 +167,7 @@ module SecondFactorManager
       self,
       security_key_credential,
       challenge: DiscourseWebauthn.challenge(self, secure_session),
-      rp_id: DiscourseWebauthn.rp_id(self, secure_session),
+      rp_id: DiscourseWebauthn.rp_id,
       origin: Discourse.base_url,
     ).authenticate_security_key
   end