fix(capacitor-ios): guard AppTransaction.shared with #available(iOS 16.0)

AppTransaction is only available from iOS 16.0 but refreshReceipts was
not wrapped in an availability check, causing build failure when the
deployment target is 15.0.

Author: j3k0

capacitor/ios/Sources/PurchasePlugin/PurchasePlugin.swift

@@ -241,19 +241,23 @@ public class PurchasePlugin: CAPPlugin, CAPBridgedPlugin {
 
     @objc func refreshReceipts(_ call: CAPPluginCall) {
         Task {
-            do {
-                let appTransaction = try await AppTransaction.shared
-                switch appTransaction {
-                case .verified(let transaction):
-                    call.resolve(["receipt": [
-                        "bundleIdentifier": transaction.bundleID,
-                        "appVersion": transaction.appVersion,
-                    ]])
-                case .unverified(_, _):
-                    call.reject("App transaction verification failed")
+            if #available(iOS 16.0, *) {
+                do {
+                    let appTransaction = try await AppTransaction.shared
+                    switch appTransaction {
+                    case .verified(let transaction):
+                        call.resolve(["receipt": [
+                            "bundleIdentifier": transaction.bundleID,
+                            "appVersion": transaction.appVersion,
+                        ]])
+                    case .unverified(_, _):
+                        call.reject("App transaction verification failed")
+                    }
+                } catch {
+                    call.reject("Failed to refresh receipts: \(error.localizedDescription)")
                 }
-            } catch {
-                call.reject("Failed to refresh receipts: \(error.localizedDescription)")
+            } else {
+                call.reject("refreshReceipts requires iOS 16.0 or newer")
             }
         }
     }