CI Workflow: Add all GKE supported k8s version

[brlbil]

The current conformance tests run against one version of k8s either a specific version or the default version that is provided by GCP.

This commit adds all the supported k8s versions by GCP in a matrix strategy.

Normal test runs:
✅ conformance-externalworkloads-v1.11.yaml
✅ conformance-externalworkloads-v1.12.yaml
✅ conformance-externalworkloads-v1.13.yaml
✅ conformance-externalworkloads.yaml
✅ conformance-gke-v1.11.yaml
✅ conformance-gke-v1.12.yaml
✅ conformance-gke-v1.13.yaml
✅ conformance-gke.yaml

Simulated scheduled runs:
✅ conformance-externalworkloads-v1.11.yaml
✅ conformance-externalworkloads-v1.12.yaml
✅ conformance-externalworkloads-v1.13.yaml
✅ conformance-externalworkloads.yaml
✅ conformance-gke-v1.11.yaml
✅ conformance-gke-v1.12.yaml
✅ conformance-gke-v1.13.yaml
✅ conformance-gke.yaml

Depends on: #26356

[nbusseneau]

LGTM provided we address the dependency on #26356, some whitespace nits below (non-blocking).

[bimmlerd]

AFAICT we are now spreading to many different zones - are we now more likely to fail the test if any zone has an issue?

[brlbil]

Yes, and no. These tests only run when it is triggered with a schedule event. So for normal daily runs, it will prefer the same zone as always. With schedule, it will run many clusters at the same time so it would be much more problematic to run them all in the same zone.
It may be cases where the zone affects the result of a test but also helps to verify if it is related to Cilium or k8s version by isolating the test.