MINOR: [C#] Bump System.Threading.Tasks.Extensions and System.Runtime…

….CompilerServices.Unsafe in /csharp (#44782)

Bumps [System.Threading.Tasks.Extensions](https://github.com/dotnet/maintenance-packages) and [System.Runtime.CompilerServices.Unsafe](https://github.com/dotnet/maintenance-packages). These dependencies needed to be updated together.
Updates `System.Threading.Tasks.Extensions` from 4.5.4 to 4.6.0
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Updates `System.Runtime.CompilerServices.Unsafe` from 4.7.1 to 6.1.0
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csharp/src/Apache.Arrow.Flight/Apache.Arrow.Flight.csproj

@@ -12,7 +12,7 @@
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFramework)'=='netstandard2.0'">
-    <PackageReference Include="Microsoft.Bcl.AsyncInterfaces" Version="6.0.0" />
+    <PackageReference Include="Microsoft.Bcl.AsyncInterfaces" Version="8.0.0" />
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFramework)'=='net472'">

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

@@ -14,8 +14,8 @@
   <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETStandard' or '$(TargetFramework)' == 'net462'">
     <PackageReference Include="System.Buffers" Version="4.5.1" />
     <PackageReference Include="System.Memory" Version="4.5.5" />
-    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="4.7.1" />
-    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.4" />
+    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="6.1.0" />
+    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.6.0" />
     <PackageReference Include="System.ValueTuple" Version="4.5.0" />
   </ItemGroup>