Verbose output (--verbose) breaks JSON output (--json)

# Context

* Version of iperf3: 3.11
* Hardware: ODROID-C2
* Operating system (and distribution, if any): Linux (Ubuntu 20.04 LTS)
* Other relevant information (for example, non-default compilers,
libraries, cross-compiling, etc.): N/A

# Bug Report

* Expected Behavior
With `--json` specified, the output of STDOUT should be working JSON
* Actual Behavior
With `--verbose` in conjunction with `--json`, the verbose output messages are sent to STDOUT along with JSON, breaking JSON
* Steps to Reproduce
`iperf3 --client XXX ... --verbose --json`
* Possible Solution
Output verbose messages to STDERR (at least when `--json` is used also).