Support

I think the message is self-explanatory. PHP 7.3 will become end of life in December 2021. While it is still supported, it has entered the second and final part of its lifetime where it only received critical security fixes but no regular bug fixes or minor security updates as explained in https://www.php.net/supported-versions This makes it precarious for use on a live site and you should be aware that we will drop support for it sometime in 2022 (6 to 9 months after it becomes End of Life and receives no updates whatsoever anymore).

The currently supported PHP version branch is PHP 7.4 which was first released in December 2018. That's two years ago. If your host does not offer it as an option two full years after it was first released they are inexcusable. Most likely they do offer it. Even GoDaddy is not that bad of a host. You probably need to take some action to enable it on your site which is what the message tells you to do.

Do note that PHP also released the 8.0 version branch in December 2020. We do not recommend updating to it just yet. It is a major version which introduced several backwards incompatible changes. While WordPress itself and our software fully support PHP 8.0 this is not the case for most smaller plugins and plugins which have not been updated in a while. The biggest problem is that minor issues that raised just a notice under PHP 5.x and 7.x are not fatal errors which will cause your site to halt with an error. We expect plugins to slowly start supporting PHP 8.0 over the next few months. Until then, upgrading your site to PHP 7.4 will be sufficient and will cover you until at least mid-2023.