fix: update SonarCloud action to v6

v5 has a known security vulnerability per SonarSource advisory.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: braboj

.github/workflows/sonarcloud.yml

@@ -18,6 +18,6 @@ jobs:
           fetch-depth: 0
 
       - name: SonarCloud Scan
-        uses: SonarSource/sonarqube-scan-action@v5
+        uses: SonarSource/sonarqube-scan-action@v6
         env:
           SONAR_TOKEN: ${{ secrets.SONAR_TOKEN }}