Update helm/chart-testing-action action to v2.8.0 (#520)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/helm.yml

@@ -34,7 +34,7 @@ jobs:
           check-latest: true
 
       - name: Set up chart-testing
-        uses: helm/chart-testing-action@v2.7.0
+        uses: helm/chart-testing-action@v2.8.0
 
       - name: Run chart-testing (lint)
         run: ct lint --check-version-increment=false --chart-dirs helm-chart --target-branch ${{ github.event.repository.default_branch }}