fix(ci): simplify helm lint workflow and fix Chart.yaml

- Remove ct lint step (too restrictive, requires maintainers)
- Remove chart-testing-action setup (unused)
- Add --kube-version 1.28.0 to helm template for compatibility
- Fix Chart.yaml URLs to point to stacklok/toolhive-cloud-ui
- Remove maintainers field (not required without ct lint)
- Align with toolhive operator chart structure

Author: peppescg

.github/workflows/lint-helm.yml

@@ -38,5 +38,5 @@ jobs:
 
             - name: Validate templates
               run: |
-                  helm template toolhive-cloud-ui ./helm --debug
-                  helm template toolhive-cloud-ui ./helm -f ./helm/values-dev.yaml --debug
+                  helm template toolhive-cloud-ui ./helm --kube-version 1.28.0 --debug
+                  helm template toolhive-cloud-ui ./helm -f ./helm/values-dev.yaml --kube-version 1.28.0 --debug