Improved comment for Jenkins version in pom.xml

Some inexperienced plugin authors seem to think they need to keep this "up to date", e.g. [here](https://github.com/jenkinsci/rpmsign-plugin/commit/b3fb77268f5199e9a0dcfb33b11988b6772f1a10#commitcomment-5913989), not realizing that users must be at least this tall to ride.