Update Rust edition to 2024

[ImplOfAnImpl]

The breaking changes are:

• gen is a reserved keyword now, so the raw syntax has to be uses instead - r#gen (though after the rand upgrade there are only a few places that had to be changed).
• Return-position impl Trait now implicitly captures all lifetime parameters, so an explicit use<..> has to be used where it's not what is needed.
• There were some changes in how the ref keyword can be used. Basically, redundant refs are now disallowed.
• std::env::{set_var, remove_var} are now unsafe (but note that they were never safe to use in multithreaded environments on *nix, including the case when an env var is set at the start of a tokio::main and a multithreaded runtime is used).
  There were different scenarios where we were using them:
  • To set the default value of RUST_LOG to "info". I removed that, now the default value can be passed to logger initialization functions and if it's not, then "info" will be used by default.
  • To set the default value of RUST_BACKTRACE to "full". For this I added the enable_rust_backtrace macro that must be called at the module scope, so that the env var is set before main or tokio::main is entered. It uses ctor under the hood, so I also upgraded ctor to 0.10.
    Also, backtrace wasn't enabled in api-web-server for some reason, so I enabled it there for consistency.
  • In tests. The potential real unsafety is not a huge deal in tests, but it can cause wrong behavior both in env-modifying tests and ones that read an env var (even if it's a different var) if they're run in parallel.
    I wrapped the functions in non-unsafe ones in test-utils, to avoid writing unsafe every time, and wrote a comment about how to deal with the potential problems. We only have 2 env-modifying tests - in logging and utils, and I turned them into integration tests, to ensure they're not run in parallel with anything else.

Additionally, if let ... can now be chained with other conditions; though not a breaking change, clippy insists that all such calls should be chained, so I indulged.

P.S.

• In "rustfmt.toml" I added style_edition = "2021" to prevent it from reformatting the code all over the place. I'll remove it and reformat the code in a follow-up PR.
• Unrelated change: I also did cargo update openssl to address some of the dependabot errors.