Support for Maven reproducible builds

[basil]

https://maven.apache.org/guides/mini/guide-reproducible-builds.html notes that maven-release-plugin version 3.0.0-M1 or later will automatically update the project.build.outputTimestamp property in pom.xml during the release in the same commit that updates version but that "if you have a custom release process tooling, you'll need to add the feature to your release tooling." This issue is blocking progress on jenkinsci/plugin-pom#919. One solution could be to set this to the timestamp of the most recent commit.