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

Bumps [System.Runtime.CompilerServices.Unsafe](https://github.com/dotnet/maintenance-packages) and [System.Threading.Tasks.Extensions](https://github.com/dotnet/maintenance-packages). These dependencies needed to be updated together.

Updates `System.Runtime.CompilerServices.Unsafe` from 6.1.0 to 6.1.1
- [Release notes](https://github.com/dotnet/maintenance-packages/releases)
- [Commits](https://github.com/dotnet/maintenance-packages/commits)

Updates `System.Threading.Tasks.Extensions` from 4.6.0 to 4.6.1
- [Release notes](https://github.com/dotnet/maintenance-packages/releases)
- [Commits](https://github.com/dotnet/maintenance-packages/commits)

---
updated-dependencies:
- dependency-name: System.Runtime.CompilerServices.Unsafe
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: System.Threading.Tasks.Extensions
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

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="8.0.0" />
+    <PackageReference Include="Microsoft.Bcl.AsyncInterfaces" Version="9.0.2" />
   </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="4.6.0" />
     <PackageReference Include="System.Memory" Version="4.6.0" />
-    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="6.1.0" />
-    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.6.0" />
+    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="6.1.1" />
+    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.6.1" />
     <PackageReference Include="System.ValueTuple" Version="4.5.0" />
   </ItemGroup>