[.NET 7] Use new Activity and ActivityContext APIs

[timbussmann]

https://github.com/Particular/NServiceBus/pull/6585 had to use some code that can be simplified with the new APIs available in .NET 7:

• https://github.com/dotnet/runtime/issues/42575 brings a ActivityContext.TryParse overload that offers an isRemote parameter that can be used to replace the workaround here: https://github.com/Particular/NServiceBus/blob/master/src/NServiceBus.Core/OpenTelemetry/Tracing/ActivityFactory.cs#L32
• Activity.HasRemoteParent can be used in related unit tests to verify that the remote flag is correctly set and ensure this behavior is properly tested.