Only add /k8s to kubeconfig server if experimental

[hasheddan]

Description of your changes

Fixes an issue where /k8s was incorrectly being appended to proxy URL when not using MCP experimental mode.

Signed-off-by: hasheddan georgedanielmangum@gmail.com

I have:

• [x] Read and followed Upbound's contribution process.
• [x] Run make reviewable to ensure this PR is ready for review.
• [ ] Added backport release-x.y labels to auto-backport this PR, as appropriate.

How has this code been tested

🤖 (up) KUBECONFIG=test.yaml up ctp kubeconfig get --mcp-experimental exp-test --token=something
🤖 (up) KUBECONFIG=test.yaml up ctp kubeconfig get e50b0bb0-48e9-4f1a-811b-f6a265c2afa9 --token=something
🤖 (up) cat test.yaml 
apiVersion: v1
clusters:
- cluster:
    server: https://proxy.upbound.io/controlPlanes/e50b0bb0-48e9-4f1a-811b-f6a265c2afa9
  name: upbound-e50b0bb0-48e9-4f1a-811b-f6a265c2afa9
- cluster:
    server: https://proxy.upbound.io/v1/controlPlanes/hasheddan/exp-test/k8s
  name: upbound-hasheddan-exp-test
contexts:
- context:
    cluster: upbound-e50b0bb0-48e9-4f1a-811b-f6a265c2afa9
    user: upbound-e50b0bb0-48e9-4f1a-811b-f6a265c2afa9
  name: upbound-e50b0bb0-48e9-4f1a-811b-f6a265c2afa9
- context:
    cluster: upbound-hasheddan-exp-test
    user: upbound-hasheddan-exp-test
  name: upbound-hasheddan-exp-test
current-context: upbound-e50b0bb0-48e9-4f1a-811b-f6a265c2afa9
kind: Config
preferences: {}
users:
- name: upbound-e50b0bb0-48e9-4f1a-811b-f6a265c2afa9
  user:
    token: something
- name: upbound-hasheddan-exp-test
  user:
    token: something

[ezgidemirel]

thanks for the quick fix @hasheddan!