How to write Pipeline to discard old builds?

For Scripted Pipelines use:

properties([
    buildDiscarder(logRotator(daysToKeepStr: '3', numToKeepStr: '3')),
])

You can use the properties method which, nested within the BuildDiscarderProperty eventually has the key you want to set. I still don't have a solid way to look up the correct syntax of each key. After much guessing and checking:

properties([[$class: 'BuildDiscarderProperty', strategy: [$class: 'LogRotator', artifactDaysToKeepStr: '', artifactNumToKeepStr: '', daysToKeepStr: '', numToKeepStr: '10']]]);

Note that this snippet is for scripted syntax.

As for declarative syntax, you can use the options block:

pipeline {
  options {
    buildDiscarder(logRotator(numToKeepStr: '30', artifactNumToKeepStr: '30'))
  }
  ...
}

Parameters for logRotator (from the source code):

• daysToKeepStr: history is only kept up to this days.
• numToKeepStr: only this number of build logs are kept.
• artifactDaysToKeepStr: artifacts are only kept up to this days.
• artifactNumToKeepStr: only this number of builds have their artifacts kept.

More information can be found in Cloudbees knowledge base and in the docs for options block.