[internal] upgrade to Rust 2021 Edition

[tdyas]

Use the Rust 2021 Edition for all crates. Upgrades the use of cargo-ensure-prefix in build-support/bin/check_rust_target_headers.sh to support Rust 2021 edition.

[tdyas]

The Rust prefix check in ./build-support/bin/check_rust_target_headers.sh is failing because cargo-ensure-prefix is using a version of the cargo crate that is too old to recognize that 2021 is a valid edition:

  Installing /home/runner/.cargo/bin/cargo-ensure-prefix
   Installed package `cargo-ensure-prefix v0.1.3` (executable `cargo-ensure-prefix`)
Error parsing /home/runner/work/pants/pants/src/rust/engine/Cargo.toml
Rust targets didn't have correct prefix:

https://github.com/pantsbuild/pants/runs/4233692777?check_suite_focus=true#step:18:522

Verified by trying to upgrade cargo-ensure-prefix to 2021 edition to see if its own unit tests could handle that change. They couldn't:

---- test_list_paths::single_package_multiple_explicit_targets stdout ----
thread 'test_list_paths::single_package_multiple_explicit_targets' panicked at 'called `Result::unwrap()` on an `Err` value: ErrorMessage { msg: "supported edition values are `2015` or `2018`, but `2021` is unknown" }

failed to parse the `edition` key

failed to parse manifest at `/Users/tdyas/TC/cargo-ensure-prefix/Cargo.toml`', src/main.rs:230:70
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

cc @illicitonion

[tdyas]

Upstream fix for cargo ensure-prefix: illicitonion/cargo-ensure-prefix#2

I wonder if we should eventually just replace cargo ensure-prefix and our integration test which checks file headers with a pants linter.