Use actor isolation for SubscriptionFeatureMappingCache (#1113)

**Required**:

Task/Issue URL:
https://app.asana.com/0/414709148257752/1208887772407026/f
iOS PR: duckduckgo/iOS#3670
macOS PR: 
What kind of version bump will this require?: Hotfix


**Description**:
Use actor isolation for `SubscriptionFeatureMappingCache`

---
###### Internal references:
[Software Engineering
Expectations](https://app.asana.com/0/59792373528535/199064865822552)
[Technical Design
Template](https://app.asana.com/0/59792373528535/184709971311943)

Sources/Subscription/SubscriptionFeatureMappingCache.swift

@@ -23,13 +23,11 @@ public protocol SubscriptionFeatureMappingCache {
     func subscriptionFeatures(for subscriptionIdentifier: String) async -> [Entitlement.ProductName]
 }
 
-public final class DefaultSubscriptionFeatureMappingCache: SubscriptionFeatureMappingCache {
+public actor DefaultSubscriptionFeatureMappingCache: SubscriptionFeatureMappingCache {
 
     private let subscriptionEndpointService: SubscriptionEndpointService
     private let userDefaults: UserDefaults
 
-    private var subscriptionFeatureMapping: SubscriptionFeatureMapping?
-
     public init(subscriptionEndpointService: SubscriptionEndpointService, userDefaults: UserDefaults) {
         self.subscriptionEndpointService = subscriptionEndpointService
         self.userDefaults = userDefaults
@@ -96,7 +94,7 @@ public final class DefaultSubscriptionFeatureMappingCache: SubscriptionFeatureMa
 
     static private let subscriptionFeatureMappingKey = "com.duckduckgo.subscription.featuremapping"
 
-    dynamic var storedFeatureMapping: SubscriptionFeatureMapping? {
+    var storedFeatureMapping: SubscriptionFeatureMapping? {
         get {
             guard let data = userDefaults.data(forKey: Self.subscriptionFeatureMappingKey) else { return nil }
             do {