How to delete old Jenkins builds on repository branches

Certain job properties are adjusted using directives in Jenkinsfiles and not the configuration in the web UI. In your Jenkinsfile:

properties([
  // only keep 25 builds to prevent disk usage from growing out of control
  buildDiscarder(
    logRotator(
      artifactDaysToKeepStr: '', 
      artifactNumToKeepStr: '', 
      daysToKeepStr: '', 
      numToKeepStr: '25',
    ),
  ),
])

You may of course adjust the various parameters to suit your needs.