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GH-45316: [Release][C#] Use GitHub Releases for artifacts (#45317)

### Rationale for this change

We don't need to use Artifactory for NuGet packages because it's not
published to Artifactory.

### What changes are included in this PR?

* Use GitHub Releases for easy to maintain
* Use `.github/workflows/csharp.yml` instead of nightly job for easy to
maintain

### Are these changes tested?

Almost yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #45316