Updated --k8s-version in LKE test

Currently, this test is failing with an "k8s_version,k8s_version is not valid".
Sure enough, it looks like 1.19 was retired recently.

This change updates the tests to use 1.21, the most recent version.  In
the future they may even need to get this value dynamically.

Author: Dorthu

test/lke/clusters.bats

@@ -27,7 +27,7 @@ teardown() {
         --node_pools.type g6-standard-1 \
         --node_pools.count 1 \
         --node_pools.disks '[{"type":"ext4","size":1024}]' \
-        --k8s_version 1.19 \
+        --k8s_version 1.21 \
         --text \
         --delimiter "," \
         --no-headers \