Disallow unmaintained crates via cargo-deny

The only unmaintained warning we had was for dotenv and an upcoming
version of cargo-deny will always error for unmaintained dependencies
(removing the configuration option) so might as well set it to deny now.

Author: svix-jplatte

deny.toml

@@ -9,7 +9,7 @@ targets = [
 db-path = "~/.cargo/advisory-db"
 db-urls = ["https://github.com/rustsec/advisory-db"]
 vulnerability = "deny"
-unmaintained = "warn"
+unmaintained = "deny"
 yanked = "deny"
 notice = "warn"
 ignore = [