ci(dependabot): remove reviewers

This option has been deprecated:
https://github.blog/changelog/2025-04-29-dependabot-reviewers-configur
ation-option-being-replaced-by-code-owners/

Refs: RATYK-107

Author: danipran

.github/dependabot.yml

@@ -8,5 +8,3 @@ updates:
       prefix: "deps"
     # Disable version updates and only allow security updates
     open-pull-requests-limit: 0
-    reviewers:
-      - "City-of-Helsinki/ratkaisutoimiston-frontend"