LKE: deprecate output variable dashboard_url

[isimluk]

LKE dashboard is being removed. This PR deprecates the dashboard_url output of linode_lke_cluster resource. Upgrades for existing users will look like the following:

  $ terraform apply
  Terraform detected the following changes made outside of Terraform since the last "terraform apply" which may have affected this plan:

    # linode_lke_cluster.test has changed
    ~ resource "linode_lke_cluster" "test" {
        - dashboard_url = "https://f59fc4f4-f580-4ee4-b966-fa2a810937b7.redacted" -> null
          id            = "123456"
          tags          = [
              "lke",
              "tf",
          ]
          # (10 unchanged attributes hidden)
          # (2 unchanged blocks hidden)
      }

  Unless you have made equivalent changes to your configuration, or ignored the relevant attributes using ignore_changes, the following plan may include actions to undo or respond to these changes. ───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────

  Changes to Outputs: ~ dashboard_url  = "https://f59fc4f4-f580-4ee4-b966-fa2a810937b7.redacted" -> ""

LKE’s Kubernetes Dashboard will be decommissioned in May 2026

Following the upstream archival of the Kubernetes Dashboard project, Akamai Cloud will decommission the LKE-hosted Kubernetes Dashboard. This change will occur in May 2026. Once decommissioned, the Kubernetes Dashboard UI will no longer be available for new and existing LKE clusters.

https://techdocs.akamai.com/cloud-computing/docs/an-overview-of-the-kubernetes-dashboard-on-lke

Context

The API endpoint is deprecated and it will start returning empty string instead of dashboard url in May 2026. Meaning, the older provider versions will continue to work fine for a forseeable future.

[lgarber-akamai]

Thank you for the contribution! Could all relevant test files be updated to no longer assert on dashboard_url (or assert that it's null)?

[isimluk]

Is there a way for me to see which those are?