Could not serialize nested ConcurrentDictionary to json

The following code works in .NET Core 3.0.
In .NET Core 3.1, it will throw `System.NotSupportedException`.
```CSharp
string input = "{\"x\":{\"y\":\"z\"}}";
var test = JsonSerializer.Deserialize<ConcurrentDictionary<string, ConcurrentDictionary<string, string>>>(input); // ok
string output = JsonSerializer.Serialize(test); // throws
```