nonisolated(unsafe) is only available in Swift 6

Author: Guilherme Souza

Sources/Storage/StorageFileApi.swift

@@ -51,9 +51,13 @@ enum FileUpload {
 }
 
 #if DEBUG
-  // It is safe to mark it as unsafe, since this property is only used in tests for overriding the
-  // boundary value, instead of using the random one.
-  nonisolated(unsafe) var testingBoundary: String?
+  #if compiler(>=6)
+    // It is safe to mark it as unsafe, since this property is only used in tests for overriding the
+    // boundary value, instead of using the random one.
+    nonisolated(unsafe) var testingBoundary: String?
+  #else
+    var testingBoundary: String?
+  #endif
 #endif
 
 /// Supabase Storage File API