NotSupportedException: Extended authentication handler is not yet supported

### Describe the bug
While connecting to emqx we often get the "Extended authentication handler is not yet supported" exception. My [library](https://github.com/BEagle1984/silverback) automatically retries after 500ms and it eventually successfully connects.
Of course, no setting is being changed in between the retries.

### Which component is your bug related to?
- Client

### To Reproduce
Steps to reproduce the behavior:
1. Using MQTTnet 4.2.1.781
2. Try to connect to EMQX from a couple of pods running on AWS EKS.
3. The first try often fails with a `NotSupportedException` (_Extended authentication handler is not yet supported_)

### Expected behavior
The connection always succeeds.

### Additional context / logging

This is the exception we get:
```batch
MQTTnet.Adapter.MqttConnectingFailedException: Error while authenticating. Extended authentication handler is not yet supported
---> System.NotSupportedException: Extended authentication handler is not yet supported
at MQTTnet.Client.MqttClient.Authenticate(IMqttChannelAdapter channelAdapter, MqttClientOptions options, CancellationToken cancellationToken)
--- End of inner exception stack trace ---
at MQTTnet.Client.MqttClient.Authenticate(IMqttChannelAdapter channelAdapter, MqttClientOptions options, CancellationToken cancellationToken)
at MQTTnet.Client.MqttClient.ConnectInternal(IMqttChannelAdapter channelAdapter, CancellationToken cancellationToken)
at MQTTnet.Client.MqttClient.ConnectAsync(MqttClientOptions options, CancellationToken cancellationToken)
at MQTTnet.Client.MqttClient.ConnectAsync(MqttClientOptions options, CancellationToken cancellationToken)
at Silverback.Messaging.Broker.Mqtt.MqttClientWrapper.TryConnectClientAsync(Boolean isFirstTry, CancellationToken cancellationToken)
```

It's not thrown for all clients instantiated within the same application and not always for the same ones.

### Code example

The actual code uses [Silverback](https://github.com/BEagle1984/silverback), which wraps MQTTnet. The resulting MQTTnet related code is something like this:
```csharp
IMqttClient mqttClient = MqttFactory.CreateClient();
await mqttClient.ConnectAsync(options, cancellationToken);
```

The options passed look like this:
```batch
{MQTTnet.Client.MqttClientOptions}
AllowPacketFragmentation: true
AuthenticationData: null
AuthenticationMethod: null
ChannelOptions: {mqtt:1883}
CleanSession: true
ClientId: "Xyz_Xyz_9a3c0f23-70e7-43f2-b266-907fafb19371"
Credentials: {MQTTnet.Client.MqttClientCredentials}
ExtendedAuthenticationExchangeHandler: null
KeepAlivePeriod: {00:00:15}
MaximumPacketSize: 0
ProtocolVersion: V500
ReceiveMaximum: 0
RequestProblemInformation: true
RequestResponseInformation: false
SessionExpiryInterval: 0
ThrowOnNonSuccessfulConnectResponse: true
Timeout: {00:01:40}
TopicAliasMaximum: 0
TryPrivate: true
UserProperties: {Count = 0}
ValidateFeatures: true
WillContentType: null
WillCorrelationData: null
WillDelayInterval: 0
WillMessageExpiryInterval: 0
WillPayload: null
WillPayloadFormatIndicator: Unspecified
WillQualityOfServiceLevel: AtMostOnce
WillResponseTopic: null
WillRetain: false
WillTopic: null
WillUserProperties: {Count = 0}
WriterBufferSize: 4096
WriterBufferSizeMax: 65535
```

The guid in the ClientId is the Kubernetes pod id.