Stabilize parallel proof runner

[zhassan-aws]

Requested feature: cargo kani has an unstable -j option to verify harnesses in parallel that was introduced in #1726. This option is currently unstable. This is a tracking issue for stabilizing it.
Use case: Improve verification performance
Link to relevant documentation (Rust reference, Nomicon, RFC):

[carolynzech]

Note: see https://model-checking.github.io/kani/rfc/intro.html for stabilization instructions.