Finish obsoletion of UseTransport<T> API

[bording]

NServiceBus 8 introduced a new transport API, but the older UseTransport<T> is still available and has not been marked as obsolete.

We don't want to remove this older API until we have equivalent new APIs for configuring persistence and serialization.

Once these new APIs have been created, and we have alignment on how all of our downstream configuration APIs work, we can start the obsoletion process, with a warning in the current NServiceBus major version, error in major version + 1, and removal in major version + 2.

The existing UseTransport<T> APIs have been marked with a PreObsolete attribute pointing to this issue.