Fix Dependabot configuration syntax error

- Remove invalid security update-types grouping
- Security updates will still be prioritized automatically by Dependabot
- Keep Jekyll grouping for related dependency updates

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ulfarsson

.github/dependabot.yml

@@ -18,9 +18,6 @@ updates:
           - "jekyll*"
         exclude-patterns:
           - "jekyll-theme-*"
-      security:
-        update-types:
-          - "security"
     # Auto-merge security patches and patch-level updates
     open-pull-requests-limit: 5
     reviewers: