Revert "Disallow unmaintained crates via cargo-deny"

This reverts commit 6f1a3d0.

We actually have an unmaintained dependency in the bridge dependency
graph that we can't easily fix right now. brige-security CI didn't run
when the commit being reverted was introduced.

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 = "deny"
+unmaintained = "warn"
 yanked = "deny"
 notice = "warn"
 ignore = [