Remove relevant interface methods from < NET6.0 targets

The implementations for these two methods use parts of the GraphQL serializer which are only available when targeting .NET 6.0 or greater.

ShopifySharp/Services/Graph/IGraphService.cs

@@ -55,6 +55,7 @@ public interface IGraphService : IShopifyService
         /// <returns>A JsonElement containing the data from the request.</returns>
         Task<JsonElement> SendAsync(string graphqlQuery, int? graphqlQueryCost = null, CancellationToken cancellationToken = default);
 
+#if NET6_0_OR_GREATER
         /// <summary>
         /// Executes a Graph API Call.
         /// </summary>
@@ -78,5 +79,6 @@ public interface IGraphService : IShopifyService
         /// <param name="cancellationToken">Cancellation Token</param>
         /// <returns>A <see cref="TResult"/> containing the data from the request.</returns>
         Task<TResult> SendAsync<TResult>(string graphqlQuery, int? graphqlQueryCost = null, CancellationToken cancellationToken = default) where TResult : class;
+#endif
     }
-}
+}