update project.build.outputTimestamp property on prepare

as part of native Maven (binary) Reproducible Builds , project.build.outputTimestamp property defines the timestamp of archive entries.
The exact value of this timestamp is not absolutely necessary to achieve the reproducibility, but if the timestamp can be the time of release, it will give a good meaning to this timestamp

release:prepare is ideal for that
https://maven.apache.org/maven-release/maven-release-plugin/examples/prepare-release.html

• ...
• Change the version in the POMs from x-SNAPSHOT to a new version (you will be prompted for the versions to use)
• Transform the SCM information in the POM to include the final destination of the tag
• update project.build.outputTimestamp property with the value of "now"
• Run the project tests against the modified POMs to confirm everything is in working order

see rewrite-poms-for-release phase as example https://maven.apache.org/maven-release-archives/maven-release-2.5.3/maven-release-manager/apidocs/org/apache/maven/shared/release/phase/RewritePomsForReleasePhase.html