fix(ske-encrypted-volumes): reference correct kubeconfig resource

[dieg666]

The kubernetes provider referenced stackit_ske_kubeconfig.example, but the resource is declared as default; terraform validate failed with Reference to undeclared resource.

Description

The kubernetes provider referenced stackit_ske_kubeconfig.example, but the
resource is declared as stackit_ske_kubeconfig.default in main.tf, so the
example fails terraform validate with "Reference to undeclared resource".
This PR aligns the four references to the actual resource name.

Checklist

• The CI pipeline passed successfully.

[h3adex]

Ty. Will add tf_validate to pre-commit.