Use 2021 edition.

Author: astonbitecode

rust/Cargo.toml

@@ -9,7 +9,7 @@ repository = "https://github.com/astonbitecode/j4rs"
 readme = "README.md"
 categories = ["api-bindings"]
 build = "build.rs"
-edition = "2018"
+edition = "2021"
 
 [badges]
 travis-ci = { repository = "astonbitecode/j4rs", branch = "master" }