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>

csharp/src/Apache.Arrow/Apache.Arrow.csproj

@@ -14,7 +14,7 @@
     <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="4.5.2" />
     <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.2" />
 
-    <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0-beta2-19270-01" PrivateAssets="All" />
+    <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0" PrivateAssets="All" />
   </ItemGroup>
 
   <ItemGroup>