"unused manifest key" warning occurs upon cargo check

[gretchenfrage]

Running the main branch (commit 08ee9ee471b61fe2c7a7248dfb628b0edcf6aca9) with the latest cargo version (cargo 1.81.0 (2dbb1af80 2024-08-20), rustc 1.81.0 (eeb90cda1 2024-09-04)) emits the following warning:

warning: /home/phoenix/repos/quinn/quinn-udp/Cargo.toml: unused manifest key: target.cfg(any(target_os = "linux", target_os = "windows")).bench