chore: upgrade edition to 2021

Author: ana

Cargo.toml

@@ -2,7 +2,7 @@
 name = "rustlings"
 version = "4.6.0"
 authors = ["anastasie <ana@ana.st>", "Carol (Nichols || Goulding) <carol.nichols@gmail.com>"]
-edition = "2018"
+edition = "2021"
 
 [dependencies]
 argh = "0.1.4"