fix(marketplace): update apiVersion in mainTemplate.json per cpa verify failure

[vdice]

Updates the Microsoft.ContainerService/managedClusters apiVersion to a more recent version to fix the cpa verify error seen here. It now matches the version we're already using for the Microsoft.KubernetesConfiguration/extensions resource.

[vdice]

The smoke test is failing due to upstream Spin Operator changing their CRD domain name on the main branch from core.spin-operator.dev to core.spinkube.dev (but the forthcoming v0.4.0 Spin Operator release which incorporates these changes isn't yet out or available for us to upgrade). In #29 I've addressed this by pinning the spin-operator version (to v0.3.0). We can safely ignore this for the template change in this PR.

error: resource mapping not found for name: "simple-spinapp" namespace: "" from "https://raw.githubusercontent.com/spinkube/spin-operator/main/config/samples/simple.yaml": no matches for kind "SpinApp" in version "core.spinkube.dev/v1alpha1"