Updates the changelog to reflect the fact that viruses mutating through radiation has been removed for a few months now

Created by: 9600bauds

Since #6495 was never added to changelogs, a lot of people never found out about it. Many people both in-game and in the thread still believe that "beneficial" viruses will turn into 40 variations of gibbingtons the moment a radstorm hits the station, as it used to be. I think it's important that the changelog is updated for changes as big as these. Properly credited to @Wizardcrying Fixes #6492 (closed)