`UseBuiltInTrxReporter` should default to true

We don't document this configuration option anywhere, yet it is essential to making sure that MNTR output is readable in environments like Azure DevOps:

```json
{
"useBuiltInTrxReporter":true
}
```

We hackily inject it into the main Akka.NET project via FAKE today:

https://github.com/akkadotnet/akka.net/blob/5d8965bc82b4b37e67679a03f67b18e22891dc2d/build.fsx#L317-L319

Start by tracing the `useBuiltInTrxReporter` configuration and compare its current default with the value injected in `build.fsx` at lines 317-319. Confirm the default is true and verify that MNTR output remains readable in the Azure DevOps scenario described.

Written by the indexing model from the issue text.