CI and repo maintenance

[robgjansen]

• Rust was already updated in Upgrade CI to rust 1.75 #3257
• Codecov access has now been revoked (as decided in Doc improvements before the NSF project wrap-up #3143)

Closes #3197

[robgjansen]

@stevenengler any hints on how to fix https://github.com/shadow/shadow/actions/runs/7380020507/job/20076929685?pr=3264#step:5:328 ?

Adding the ref & as recommended by the compiler yields

the trait bound `i32: AsFd` is not satisfied
the following other types implement trait `AsFd`:
  Box<T>
  File
  PollFd<'fd>
  Stdin
  StdinLock<'a>
  Stdout
  StdoutLock<'a>
  Stderr
and 23 othersrustc[Click for full compiler diagnostic](rust-analyzer-diagnostics-view:/diagnostic%20message%20%5B2%5D?2#file%3A%2F%2F%2Fhome%2Frjansen%2Fdev%2Fshadow%2Fsrc%2Ftest%2Ftest_utils.rs)
test_utils.rs(296, 25): required by a bound introduced by this call
poll.rs(39, 20): required by a bound in `PollFd::<'fd>::new`

[stevenengler]

Yeah that would be a pain to fix. See #3141 #3265.

I would just manually edit src/tests/Cargo.toml and set the nix version back to what it was originally. You might need to also run cargo update again to update the lock file.

[robgjansen]

Before merging, I will remove fedora:37 and tor-0.4.8.7 from the required checks, and change fedora:38 and tor-0.4.8.10 from optional to required.