Add support for testing a net6 client against a net48 service over websockets.

[LukeButters]

Halibut tests only test communication between the same dotnet version (e.g. net6 to net6 or net48 to net48).

WebSockets services are only supported in net48 (while the client is supported in both), which means websock tests only run when testing net48 to net48. This means we have no tests for websocket code in net6.

We ought to test that a net6 client can communicate with a net48 service to cover this gap.