Pin k8s image version in integration tests (#3461)

Previously, the integration tests would rely on the default k8s
image version, which differs based on the version of kind we're
using. This commit explicitly pins the image version such that 1)
it's clear what version of k8s is in use, and 2) it's easily
changed when needed.

Signed-off-by: Evan Gilman <[email protected]>

Author: evan2645

doc/supported_integrations.md

@@ -18,5 +18,5 @@ the last minor version that supports it (v1.16).
 
 ## Kubernetes
 
-The SPIRE project currently supports Kubernetes 1.18 through 1.20. Later
+The SPIRE project currently supports Kubernetes 1.18 through 1.21. Later
 versions may also work but are not explicitly exercised by integration tests.

test/integration/common

@@ -203,12 +203,14 @@ download-kubectl() {
 }
 
 start-kind-cluster() {
+    K8SIMAGE=kindest/node:v1.21.1@sha256:fae9a58f17f18f06aeac9772ca8b5ac680ebbed985e266f711d936e91d113bad
+
     local kind_path=$1
     local kind_name=$2
     local kind_config_path=$3
 
     log-info "starting cluster..."
-    "${kind_path}" create cluster --name "${kind_name}" --config "${kind_config_path}" || fail-now "unable to create cluster"
+    "${kind_path}" create cluster --name "${kind_name}" --config "${kind_config_path}" --image "${K8SIMAGE}" || fail-now "unable to create cluster"
 }
 
 load-images() {