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ARROW-8203: [C#] Use the latest SourceLink

This fixes "dotnet pack". See JIRA issue for full error message.

Closes #6711 from kou/ci-release-csharp

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>