restrict new onboarding to macOS 13 and above (#3716)

Task/Issue URL: https://app.asana.com/0/0/1209114783599171/f

**Description**: Will limit the onboarding experiment to macOS version
from 13 and above. This is only temporary until we merge the new C-S-S
version containing the fixed FE app.

DuckDuckGo/Tab/Model/Tab.swift

@@ -795,9 +795,8 @@ protocol NewWindowPolicyDecisionMaker {
             return
         }
 #endif
-
-        if PixelExperiment.cohort == .newOnboarding {
-            setContent(.onboarding)
+        if #available(macOS 13, *) {
+            setContent(PixelExperiment.cohort == .newOnboarding ? .onboarding : .onboardingDeprecated)
         } else {
             setContent(.onboardingDeprecated)
         }