Warn about unrecognized fields in config.json

[nikurt]

We already use serde_ignored to detect unrecognized fields, but there is a catch.
It only notifies about unrecognized fields in sub-fields of Config.
Because field Config::gc has a serde attribute flatten, that crate fails to detect unrecognized fields in Config itself, only detects unrecognized fields in its sub-fields.

The issue is known in serde_ignored but wasn't fixed for more than a year:
dtolnay/serde-ignored#10