2021 edition upgrade; MSRV 1.56 (#343)

Changes the `edition` to 2021 in both the `yubikey` and `yubikey-cli`
crates.

Removes `TryFrom`/`TryInto` imports, now that they're in the prelude.

Cargo.lock

@@ -894,7 +894,7 @@ dependencies = [
 
 [[package]]
 name = "yubikey"
-version = "0.5.0"
+version = "0.6.0-pre"
 dependencies = [
  "chrono",
  "cookie-factory",
@@ -928,7 +928,7 @@ dependencies = [
 
 [[package]]
 name = "yubikey-cli"
-version = "0.5.0"
+version = "0.6.0-pre"
 dependencies = [
  "env_logger",
  "gumdrop",