Use 'npm' for yarn packages.

According to
https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#package-ecosystem
'npm' should also be used for yarn.

.github/dependabot.yml

@@ -5,7 +5,7 @@ updates:
     schedule:
       interval: "daily"
 
-  - package-ecosystem: "yarn"
+  - package-ecosystem: "npm"
     directory: "/"
     schedule:
       interval: "daily"