Add validation for package bundle kubernetes version and update docs

Currently when a customer upgrades the kubernetes version for their cluster, we don't check if the packages bundle has the same kubernetes version as the cluster. We should add a validation to block the customer from upgrading their cluster if the kubernetes version in their cluster spec doesn't match the kubernetes version in the active bundle of their package bundle controller. This will force customers to [upgrade](https://anywhere.eks.amazonaws.com/docs/packages/packagebundles/#activating-a-package-bundle) their active package bundle to the latest version corresponding to their kubernetes version specified in the cluster spec. The [Upgrade cluster](https://anywhere.eks.amazonaws.com/docs/clustermgmt/cluster-upgrades/upgrade-overview/#upgrading-kubernetes-versions) section in the EKS-A docs also needs to be updated to let customers know that they need to upgrade their active package bundle too every time they upgrade their cluster to a newer kubernetes version.

This should also fix [#8984](https://github.com/aws/eks-anywhere/issues/8984)

Start by tracing the cluster upgrade validation path and the package bundle controller's active bundle, using the cluster spec's Kubernetes version as the comparison point. Read the linked package-bundle activation and Upgrade cluster documentation, then update the validation and docs so mismatched versions block upgrades and the required bundle upgrade is explained; confirm this addresses #8984.

Written by the indexing model from the issue text.