Let users pick a specific k8s version for kind bootstrap cluster

What does this MR do and why?

This MR was raised to address the issue under #1883 (closed). Users were not able to specify the Kind cluster K8S version before. With this MR, the user will be able to specify the Kind cluster K8s version directly from the CLI when running bootstrap.sh. This could be done by setting the value of the ENV variable KIND_CLUSTER_K8S_VERSION (e.g. KIND_CLUSTER_K8S_VERSION="v1.31.1") at runtime. The script tools/kind/bootstrap-cluster.sh was modified to check for the existence of the variable and create the node with the specified version. If it is not found, the script will follow the existing logic.

Related reference(s)

Closes #1883 (closed)