Use System.Text.Json as default serializer

[timbussmann]

Currently, the Azure Functions packages use the NServiceBus.Newtonsoft.Json package as a dependency and configure the Newtonsoft.Json serializer as the default serializer for function endpoints. This introduces an additional dependency on those JSON related packages which technically we might not need given that the Azure Functions packages target .NET Core 3.1 which has has a built-in JSON support (System.Text.Json).

Community projects like https://github.com/NServiceBusExtensions/NServiceBus.Json have already shown that NServiceBus can work with the built-in Json serializer instead of the Newtonsoft serializer.
It might be necessary to investigate potential compatibility issues given that existing NServiceBus endpoints might still use the Newtonsoft.Json packages instead.

[kbaley]

See also Particular/NServiceBus#6577