Enable Dependabot updates for transitive dependencies (#287)

Configure Dependabot's `allow` key with `dependency-type: all` so it
also updates transitive dependencies (those only in the lockfile), not
just direct dependencies. This removes the need to manually refresh
lockfiles.

https://docs.github.com/en/code-security/reference/supply-chain-security/dependabot-options-reference#allow--

GUS-W-24099599.

Author: edmorley

.github/dependabot.yml

@@ -4,4 +4,7 @@ updates:
     directory: "/"
     schedule:
       interval: "monthly"
+    allow:
+      # Update transitive dependencies too.
+      - dependency-type: "all"
     versioning-strategy: increase-if-necessary