Different Kirby versions support different PHP versions.
You can find a little chart here:
As you can see, Kirby 3.5.x already supports PHP 8.0, but Kirby 4 will probably drop support for PHP 8.0, because it has already reached EOL, but Kirby 3.9.x will still work with PHP 8.0, so that would be your way to go with your current server. Might make sense to consider another provider where you can choose PHP versions per domain.