as Ann already wrote, after sonar.branch.target has vanished with Sonarqube 8.x, the new code
period has to be set per branch, when previous version is the default version for every branch.
indicates you’ve hit the problem discussed here
We use a generic Jenkins pipeline for maven builds wrapped in a Jenkins shared library.
As we use Jenkins multibranch pipelines, the problem that @reitzmichnicht mentioned is no problem,
as Jenkins creates a job for every branch that fits the multibranch configuration.
You may set the new code definition via web api, see if it works for you
Finally you may vote to bring back sonar.branch.target
