This message indicates that the PHP version used by the server to process PHP is outdated.
How risky is an outdated PHP version?
If you’re only a month behind, it’s not a problem. Even a few months may not be a big issue. However, if you’re significantly behind on major PHP releases (a year or more), it becomes different. There is a risk of security vulnerabilities.
How can you update PHP to a new version?
Usually, your web host takes care of this for you. If you have a VPS (Virtual Private Server), you may need to update PHP yourself or request assistance from your web host. There may be associated costs involved.