Bug 1959542 - Enable ToS by default in Firefox 138.x mid-cycle dot release (Windows, Mac, Linux) r=hanna_a,omc-reviewers

- Show ToS modal by default for all new users on official builds
  - Remove code related to the on-train rollout for Mac/Linus/MSIX

Differential Revision: https://phabricator.services.mozilla.com/D247111

browser/components/asrouter/modules/OnboardingMessageProvider.sys.mjs

@@ -1408,7 +1408,7 @@ const BASE_MESSAGES = () => [
 
 const PREONBOARDING_MESSAGES = () => [
   {
-    id: "ON_TRAIN_ROLLOUT",
+    id: "NEW_USER_TOU_ONBOARDING",
     enabled: true,
     requireAction: true,
     currentPolicyVersion: 3,