Bump MSRV to 1.81 to match home@0.5.11

[micolous]

fido-hid-rs has a transitive build dependency on home:

% cargo tree -i home
home v0.5.9
└── which v4.4.2
    └── bindgen v0.65.1
        [build-dependencies]
        └── fido-hid-rs v0.5.0

This now fails in CI on rustc 1.79, which pulls in home@0.5.11:

error: rustc 1.79.0 is not supported by the following package:
  home@0.5.11 requires rustc 1.81
Either upgrade rustc or select compatible dependency versions with
`cargo update <name>@<current-ver> --precise <compatible-ver>`
where `<compatible-ver>` is the latest version supporting rustc 1.79.0

MSRV has been bumped by point releases in rust-lang/cargo#14867 and rust-lang/cargo#14871

home@0.5.9 is the last version to support rustc 1.79.

This would probably also be fixed by an MSRV-aware resolver.

FWIW home is "not intended for external use" .. and should use something like env-home: harryfei/which-rs#104

• cargo test has been run and passes
• documentation has been updated with relevant examples (if relevant)