chore: drop inapplicable dependabot ecosystems (#6)

.github/dependabot.yml

@@ -4,10 +4,11 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+    open-pull-requests-limit: 10
     groups:
       gomod-minor-patch:
         update-types: ["minor", "patch"]
-  - package-ecosystem: "github-actions"
-    directory: "/"
-    schedule:
-      interval: "monthly"
+      security:
+        applies-to: security-updates
+        patterns: ["*"]
+    labels: ["dependencies", "security"]