Search for more information about rkhunter. Read the manual etc. Helps you understand how it works. Plesk calls this system watchdog but in fact it is rkhunter.
Rkhunter scans files and saves/remembers how they 'look'. So if a file changes rkhunter will show a warning. If this warning is correct, because of an update or an intended change you will have to tell rkhunter the files have been changed and can be remembered in this state. Therefore you run the command rkhunter --propupd or something like: /opt/psa/admin/bin/modules/watchdog/rkhunter --propupd. Pay attention though! Only give this command when you are pretty sure your system is clean.
A good common practice is to run rkhunter --propupd on a new clean install. And run rkhunter -c --rwo (report warnings only) before an update and check for warnings. Run the update e.g. apt-get upgrade and afterwards run rkhunter --propupd again. Of course this takes time and effort!
The three version warnings probably don't really matter. Debian is not a distribution that quickly upgrades/updates, because they review extensively and therefore are really stable and secure as well. So you will often find versions to be outdated on a Debian system according to rkhunter.