Ignore nuget custom registry for dotnet-version

[mueller-ma]

How are you running Renovate?

Self-hosted Renovate

If you're self-hosting Renovate, tell us which platform (GitHub, GitLab, etc) and which version of Renovate.

39.111.0

Please tell us more about your question or problem

I configured Renovate to use a custom on-prem registry for nuget. Renovate tries to use this registry also for dotnet-version, but throws a warning then:

WARN: Custom registries are not allowed for this datasource and will be ignored
{
  "datasource": "dotnet-version"
  "registryUrls": [
    "https://example.com"
  ]
  "defaultRegistryUrls": null
}

I don't want Renovate to use a registry for dotnet-version, but I haven't found a way to configure the registry for nuget packages only.
For reproduction:

• https://github.com/mueller-ma/renovate-reproduce/blob/main/global.json
• https://github.com/mueller-ma/renovate-reproduce/blob/main/renovate.json
• Actual nuget packages aren't configured in this repo as they aren't required to demonstrate the issue.

How can I get rid of this warning?

Logs (if relevant)

Logs

DEBUG: Deleted cached dep updates
INFO: Dependency extraction complete
{
  "baseBranch": "main"
  "stats": {
    "managers": {
      "docker-compose": {
        "fileCount": 1,
        "depCount": 0
      },
      "nuget": {
        "fileCount": 1,
        "depCount": 1
      }
    },
    "total": {
      "fileCount": 2,
      "depCount": 1
    }
  }
}

WARN: Custom registries are not allowed for this datasource and will be ignored
{
  "datasource": "dotnet-version"
  "registryUrls": [
    "https://example.com"
  ]
  "defaultRegistryUrls": null
}

[rarkins]

Don't use nuget: { }. Use packageRules with matchDatasources=nuget

[mueller-ma]

I updated the docs: #33700