sys: Upgrade num_enum to 0.7

[svix-jplatte]

Currently, rdkafka pulls in syn 1.0 via num_enum 0.5. Most parts of the Rust ecosystem have moved to syn 2.0, and it's annoying to have both in the dependency tree because it's pretty heavy in terms of compile time.

I chose to first update Cargo.lock via cargo update because it was very outdated, and to show that the actual lockfile diff for the num_enum upgrade you would also see elsewhere from this doesn't pull in anything extra. I can update the PR to only do the minimal lockfile upgrade if you prefer.

I also checked for breaking changes. num_enum unfortunately doesn't have a changelog, but it seems like the only relevant changes are:

• num_enum 0.6 changed some behavior w.r.t. #[default] / #[num_enum(default)] attributes, which you aren't using
• num_enum 0.7 increases the minimum supported Rust version to 1.70.0, which matches the current MSRV here as of Update MSRV to 1.70 #722

[svix-jplatte]

Gentle ping @benesch @davidblewett (you seem to have recently reviewed other PRs)

[davidblewett]

LGTM. Will check test results later.

[svix-jplatte]

CI results mostly match main, though (7.5.1, 3.6) succeeded there.
I guess test_produce_consume_consumer_nonempty_callback is flaky?