chore(dependabot): update all Cargo manifests (#7672)

# Which issue does this PR close?

None

# Rationale for this change

There are more `Cargo.toml` files that should be considered by
Dependabot.

# What changes are included in this PR?

Search recursively for manifests instead of just the one in the root.

# Are there any user-facing changes?

No

Author: mbrobbel

.github/dependabot.yml

@@ -1,7 +1,8 @@
 version: 2
 updates:
   - package-ecosystem: cargo
-    directory: "/"
+    directories:
+      - "**/*"
     schedule:
       interval: daily
     open-pull-requests-limit: 10