Very unintuitive error message for old configs with `ping-passthrough`

Expected Behavior

Old configs with a string value for ping-passthru should throw a more intuitive error or use the string value for a preset instead.

Actual Behavior

After #1870, old configs with ping-passthru break with the very cryptic error message of:

java.lang.ClassCastException: class java.lang.String cannot be cast to class com.electronwill.nightconfig.core.Config

Steps to Reproduce

Make a config

Plugin List

Not relevant

Velocity Version

Latest, commit SHA 21bbf35d1da1d8a74aad897ecb4e6915f121898d

Additional Information

This is to be expected, as we're trying to pass a String to a parameter, that accepts Config, but it took me a good while of digging to find the culprit.

If it were up to me to fix this, I'd define some presets, which a string value can correlate to, so that old configs don't break and backwards compatibility is retained

Start by reproducing the failure with an old config using a string value for ping-passthru, then trace the handling introduced by #1870. Done means the configuration either accepts the legacy string as a preset or reports an intuitive, actionable error instead of the ClassCastException.