message: cargo: bump to 2021 edition
  This updates the Cargo manifest to use the latest Rust edition.
diff: @@ -7,7 +7,7 @@ license = "Apache-2.0"
  keywords = ["cincinnati", "coreos", "fedora", "rpm-ostree"]
  authors = ["Luca Bruno <luca.bruno@coreos.com>"]
  repository = "https://github.com/coreos/zincati"
  -edition = "2018"
  +edition = "2021"
  [dependencies]
  actix = "0.13"
  