MINOR: [C#] Bump System.Runtime.CompilerServices.Unsafe from 4.5.3 to…

… 4.7.1 in /csharp (#37074)

Bumps [System.Runtime.CompilerServices.Unsafe](https://github.com/dotnet/corefx) from 4.5.3 to 4.7.1.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a href="https://github.com/dotnet/corefx/releases">System.Runtime.CompilerServices.Unsafe's releases</a>.</em></p>
<blockquote>
<h2>.NET Core 2.1.0 RC1</h2>
<ul>
<li><a href="https://github.com/dotnet/core/blob/master/release-notes/2.1/Preview/2.1.0-rc1.md">Release Notes</a></li>
<li><a href="https://github.com/dotnet/core/blob/master/release-notes/2.1/Preview/2.1.0-rc1-known-issues.md">Known Issues</a></li>
<li><a href="https://github.com/dotnet/core/blob/master/release-notes/download-archives/2.1.0-rc1-download.md">Download and Install</a></li>
</ul>
<h2>Repos</h2>
<ul>
<li><a href="https://github.com/dotnet/coreclr/releases/tag/v2.1-rc1">CoreCLR</a></li>
<li><a href="https://github.com/dotnet/corefx/releases/tag/v2.1-rc1">CoreFX</a></li>
<li><a href="https://github.com/dotnet/core-setup/releases/tag/v2.1-rc1">Core-Setup</a></li>
<li><a href="https://github.com/dotnet/cli/releases/tag/v2.1.300-rc1-008673">CLI</a></li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li>See full diff in <a href="https://github.com/dotnet/corefx/commits">compare view</a></li>
</ul>
</details>
<br />

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Authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

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

@@ -11,7 +11,7 @@
  <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETStandard'">
  <PackageReference Include="System.Buffers" Version="4.5.1" />
  <PackageReference Include="System.Memory" Version="4.5.5" />
- <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="4.5.3" />
+ <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="4.7.1" />
  <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.4" />
  </ItemGroup>