Bump libc-test to Rust 2021 Edition

[eduardosm]

libc-test was already at 2018 Edition, bumping to 2021 just needs changing it in Cargo.toml

[rustbot]

r? @JohnTitor

rustbot has assigned @JohnTitor.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[tgross35]

Thanks!

[eduardosm]

The failure looks unrelated to this PR

#7 [internal] load metadata for docker.io/library/ubuntu:23.10
#7 ERROR: unexpected status from HEAD request to https://registry-1.docker.io/v2/library/ubuntu/manifests/23.10: 401 Unauthorized
------
 > [internal] load metadata for docker.io/library/ubuntu:23.10:
------
Dockerfile:1
--------------------
   1 | >>> FROM ubuntu:23.10
   2 |     RUN apt-get update
   3 |     RUN apt-get install -y --no-install-recommends \
--------------------
ERROR: failed to solve: ubuntu:23.10: failed to resolve source metadata for docker.io/library/ubuntu:23.10: unexpected status from HEAD request to https://registry-1.docker.io/v2/library/ubuntu/manifests/23.10: 401 Unauthorized

[tgross35]

Yeah, I'll retry. Weird that it doesn't give any sort of notification for failures...