Update edition in Cargo.toml

Author: khrs

Cargo.toml

@@ -4,7 +4,7 @@ version = "1.4.6"
 license = "Apache-2.0"
 authors = ["ING Systems <contact@ing-systems.com>"]
 description = "queue-file is a lightning-fast, transactional, file-based FIFO"
-edition = "2018"
+edition = "2021"
 homepage = "https://github.com/ing-systems/queue-file"
 repository = "https://github.com/ing-systems/queue-file"
 readme = "README.md"