Fix broken back navigation on Privacy Pro purchase page after redirect (

#2629)

Task/Issue URL:
https://app.asana.com/0/1177771139624306/1207085358710521/f

**Description**:
Fix broken back navigation on Privacy Pro purchase page after redirect

**Steps to test this PR**:
1. Go to the launch blog post preview:
https://spreadprivacy.com/meetprivacypro/
2. Click on a “duckduckgo.com/pro” link to load the PP landing page.
3. Click the browser’s back button.



---
###### Internal references:
[Pull Request Review
Checklist](https://app.asana.com/0/1202500774821704/1203764234894239/f)
[Software Engineering
Expectations](https://app.asana.com/0/59792373528535/199064865822552)
[Technical Design
Template](https://app.asana.com/0/59792373528535/184709971311943)
[Pull Request
Documentation](https://app.asana.com/0/1202500774821704/1204012835277482/f)

DuckDuckGo/Tab/Navigation/RedirectNavigationResponder.swift

@@ -40,7 +40,7 @@ struct RedirectNavigationResponder: NavigationResponder {
             let shouldHidePrivacyProDueToNoProducts = SubscriptionPurchaseEnvironment.current == .appStore && SubscriptionPurchaseEnvironment.canPurchase == false
             let isPurchasePageRedirectActive = isFeatureAvailable && !shouldHidePrivacyProDueToNoProducts
 
-            return isPurchasePageRedirectActive ? URL.subscriptionPurchase : nil
+            return isPurchasePageRedirectActive ? URL.subscriptionBaseURL : nil
         }
 
         return nil