Defaults internal Privacy Pro links to /subscriptions (#1166)

Please review the release process for BrowserServicesKit
[here](https://app.asana.com/0/1200194497630846/1200837094583426).

**Required**:

Task/Issue URL:
https://app.asana.com/0/72649045549333/1209009315184468/f
iOS PR: duckduckgo/iOS#3812
macOS PR: duckduckgo/macos-browser#3741
What kind of version bump will this require?: Patch

**Description**:
Previously, default Privacy Pro links were sent to
`/subscriptions/welcome`, and the frontend redirected to
`/subscriptions`.

Sources/Subscription/SubscriptionURL.swift

@@ -43,7 +43,7 @@ public enum SubscriptionURL {
                 URL(string: "https://duckduckgo.com/subscriptions?environment=staging")!
             }
         case .purchase:
-            SubscriptionURL.baseURL.subscriptionURL(environment: environment).appendingPathComponent("welcome")
+            SubscriptionURL.baseURL.subscriptionURL(environment: environment)
         case .faq:
             URL(string: "https://duckduckgo.com/duckduckgo-help-pages/privacy-pro/")!
         case .activateViaEmail: