Bump k8s version in kind e2e (#454)
Bump k8s version in kind e2e (#454)

This patch updates k8s version in kind e2e.

The min k8s version of knative v0.21.x is v1.18. Please refer to [Knative Serving Version Table](https://github.com/knative/community/blob/master/mechanics/RELEASE-VERSIONING-PRINCIPLES.md#knative-serving-version-table)

Eventing already uses these versions - https://github.com/knative/eventing/blob/5451f191180a584cb7c1e34079177f63927720f7/.github/workflows/kind-e2e.yaml#L23-L26
knative-sandbox/net-kourier,knative-sandbox/net-kourier