Bump MSRV to 1.59.0 and edition to 2021

We have stuck to Rust 1.56.1 for a while for supporting the oldest LTS
release of Yocto (Kirkstone). This release bumped its Rust support to
1.59.0 in March 2023.

Let's take this to our advantage and gain the option to allow for
example dependencies using Cargos named-profiles (which got stabilized
with Cargo 1.57).

Cargo.toml

@@ -5,8 +5,8 @@ authors = [
     "Bryant Mairs <[email protected]>",
     "Jesse Braham <[email protected]>",
 ]
-edition = "2018"
-rust-version = "1.56.1"
+edition = "2021"
+rust-version = "1.59.0"
 description = "A cross-platform low-level serial port library."
 documentation = "https://docs.rs/serialport"
 repository = "https://github.com/serialport/serialport-rs"