Update the Rust edition to 2021

Author: msuchane

Cargo.toml

@@ -4,7 +4,7 @@ version = "2.9.4"
 description = "Generate pre-populated module files formatted with AsciiDoc that are used in Red Hat and Fedora documentation."
 authors = ["Marek Suchánek <[email protected]>"]
 license = "GPL-3.0-or-later"
-edition = "2018"
+edition = "2021"
 documentation = "https://docs.rs/newdoc"
 readme = "README.md"
 repository = "https://github.com/mrksu/newdoc"