Fix release-drafter v7 label validation error (#317)

In v7, 'label' (singular) must be a string. The Dependency updates
category used 'label' with a list — change to 'labels' (plural).

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: gnodet

.github/release-drafter.yml

@@ -45,7 +45,7 @@ categories:
       - test
       - tests
   - title: 📦 Dependency updates
-    label:
+    labels:
       - dependencies
       - dependency
       - dependency-upgrade