The checkpoint API returns outdated information

The checkpoint API returns outdated information:

```
$ curl -s https://checkpoint-api.hashicorp.com/v1/check/consul | jq .
{
"product": "consul",
"current_version": "1.15.15",
"current_release": 1730384936,
"current_download_url": "https://releases.hashicorp.com/consul/1.15.15/",
"current_changelog_url": "https://github.com/hashicorp/releases/tag/ent-changelog-1.15.15",
"project_website": "https://www.consul.io",
"alerts": []
}
```

This makes the checks at https://github.com/hashicorp/consul/blob/main/command/agent/agent.go#L81-L121 useless.

The checkpoint API should be updated or the `startupUpdateCheck()` can be removed.