fix broken migration abstraction, fixes restart after upgrade so the runtimestate is correct, fixes sql alter column migration to actually run if its nullable.

src/Umbraco.Core/Configuration/Models/NuCacheSettings.cs

@@ -13,12 +13,10 @@ namespace Umbraco.Cms.Core.Configuration.Models
         /// </summary>
         public int? BTreeBlockSize { get; set; }
 
-        // TODO: Set default to MessagePack
-
         /// <summary>
         /// The serializer type that nucache uses to persist documents in the database.
         /// </summary>
-        public NuCacheSerializerType NuCacheSerializerType { get; set; }
+        public NuCacheSerializerType NuCacheSerializerType { get; set; } = NuCacheSerializerType.MessagePack;
 
         /// <summary>
         /// The paging size to use for nucache SQL queries.