Update rust toolchain version for Kani 0.19

[zhassan-aws]

Biweekly update for the rust toolchain version. For Kani 0.19, we will target the new year version: nightly-2023-01-01 :)

[celinval]

This is currently blocked by diffblue/cbmc#7462

[zhassan-aws]

More details about the blocker is in #2051.

[tautschnig]

diffblue/cbmc#7462 is now resolved.

[zhassan-aws]

We skipped updating the rust toolchain for Kani 0.19. Updating it for Kani 0.20 is tracked in #2113. Closing this one.