Cargo.toml: Update edition to 2021

We have recently (#104) dropped support for compilers that don't support
edition 2021. Make this explicit by enforcing edition 2021 in
Cargo.toml.

Cargo.toml

@@ -3,7 +3,7 @@ name = "avr-device"
 version = "0.3.4"
 
 authors = ["Rahix <[email protected]>"]
-edition = "2018"
+edition = "2021"
 description = "Register access crate for AVR microcontrollers"
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/Rahix/avr-device"