Make serializer configuration explicit in NServiceBus 9

[DavidBoike]

When the System.Text.Json serializer was added in NServiceBus 8.1, a logger warning was added on startup when the XmlSerializer was selected by default because no serializer was explicitly configured.

Because no message serializer was selected, the default XmlSerializer will be used instead. In a future version of NServiceBus the XmlSerializer will no longer be the default. For better forward compatibility, either choose a different message serializer, or make the choice of XML serialization explicit using endpointConfiguration.UseSerialization()

In NServiceBus 9, the logged warning should be upgraded to an exception to force all users to specify serialization. This will clear the way for a future major version beyond NServiceBus 9 to make System.Text.Json serialization the new default.

Note that when NServiceBus 8.1 was released, an explicit serializer was added to samples tied to the "Core" component, but not to other components, because they wouldn't pull in NServiceBus 8.1 when using 8.* anyway. When rolling out the next versions of samples, all samples will need to be F5-tested to ensure that they set the serializer and don't throw on startup.