Use `expect` lint level for sealed trait lints once MSRV>=1.81

Many of the arti crates use sealed traits with Sealed using the allow lint level for unreachable_pub (and in the future, possibly unnameable_types; see #1755 (comment 3135893)).

But I think we don't just want to allow the lint, but also expect the lint. If the lint doesn't trigger, then I think it's a bug (the Sealed trait has now become publicly reachable). We can enforce this using the expect lint level instead of allow. This only became available in rust 1.81, so we must wait for our MSRV to support this rust version before using expect.