Preflight validation for checking restricted node labels.

**What would you like to be added**:
Preflight validation for checking restricted node labels.

**Why is this needed**:
Users face some issues if they use kubeadm restricted node labels in the spec while creating/upgrading clusters. Introduce a preflight validation to check for the names.

Start by locating the existing preflight validation entry points and the cluster creation and upgrade paths that process node labels. Review how kubeadm restricted node-label names are represented and find the related validation tests, if any. Done means invalid restricted labels are detected before creation or upgrade and the behavior is covered by tests.

Written by the indexing model from the issue text.