app: set name of app updater route and add it to anon api routes (sou…

…rcegraph#52340)

The updater route was missing some "glue". This fixes it by:
- adding it to anon api routes
- naming the route
## Test plan
tested locally

#### No update
```
curl -o /dev/null -s -w "%{http_code}\n" "https://sourcegraph.test:3443/.api/app/check/update?target=aarch64&current_version=2023.8.1&arch=darwin"
204
```

#### Update!
```
curl -s "https://sourcegraph.test:3443/.api/app/check/update?target=aarch64&current_version=2023.2.1&arch=darwin" | jq
{
  "version": "2023.5.8+dev.123.shamd245",
  "notes": "Test version",
  "pub_date": "2023-05-07T12:25:57Z",
  "signature": "dW50cnVzdGVkIGNvbW1lbnQ6IHNpZ25hdHVyZSBmcm9tIHRhdXJpIHNlY3JldCBrZXkKUlVTWVByYmswNFZITlB3UDZaQUE1WnlrZEYxMzBZd1hNYVY5U2U0dGJmZzZNUm1aTDdoMzF3TFNoYlo5VFRiblI5NTQzV2NsS1J0TWZUSmFJQXVmejV1U1p2WXo5ZWhDMGc4PQp0cnVzdGVkIGNvbW1lbnQ6IHRpbWVzdGFtcDoxNjgzNTQzMzgyCWZpbGU6U291cmNlZ3JhcGguQXBwXzEuMC4wLWRldl94NjQuZG1nCmdGM1NDODlWNmJFdEpyUGZNd3orYzdYT1pjOXFjMFJQZXBqNG00ZC9zTC9vajBrakVzSkV5OGRIUE4zak1aSlNVZDN5eXc2YVNRNWs3elovZ3Z1bkJnPT0K",
  "url": "https://github.com/sourcegraph/sourcegraph/releases/download/untagged-03042cee5585262192cf/Sourcegraph.App_1.0.0-dev_x64.dmg"
}

```
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