Allow unknown Clippy lints with pre-1.51.0
Allow unknown Clippy lints with pre-1.51.0

Rust 1.51.0 replaced `clippy::unknown-clippy-lints` with
`unknown-lints`, but our MSRV, 1.48.0, doesn't know about that. So we'll
use both for a while.

newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,newsboat/newsboat