Fix the default teleport-kube-agent upgrade server

[wadells]

updates.gravitational.io does not currently exist, but updates.releases.teleport.dev does, and is the correct endpoint.

Tested by adding the following to a 13.1.0 agent connected to a (frontrunning) 13.1.0 cloud account.

updater:
  enabled: true
  versionServer: "https://updates.releases.teleport.dev/v1/"

[wadells]

Without this in place, using the cloud auto updater instructions results in the following error:

2023-06-06T22:16:43Z    ERROR   trigger failed to evaluate, using its default value     {"controller": "statefulset", "controllerGroup": "apps", "controllerKind": "StatefulSet", "StatefulSet": {"name":"teleport-agent","namespace":"teleport"}, "namespace": "teleport", "name": "teleport-agent", "reconcileID": "da8864bf-36c5-4b92-841c-305b17383b66", "namespacedname": "teleport/teleport-agent", "kind": "StatefulSet", "trigger": "critical update", "defaultValue": false, "error": "Get \"https://update.gravitational.io/v1/stable/cloud/critical\": dial tcp: lookup update.gravitational.io on 172.20.0.10:53: no such host"}

[public-teleport-github-review-bot]

@wadells See the table below for backport results.

Branch	Result
branch/v13	Create PR