Bump System.IO.Abstractions from 6.0.17 to 6.0.21 (#324)

Bumps [System.IO.Abstractions](https://github.com/System-IO-Abstractions/System.IO.Abstractions) from 6.0.17 to 6.0.21.
- [Release notes](https://github.com/System-IO-Abstractions/System.IO.Abstractions/releases)
- [Commits](TestableIO/System.IO.Abstractions@v6.0.17...v6.0.21)

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

Co-authored-by: null <27856297+dependabot-preview[bot]@users.noreply.github.com>

Author: dependabot-preview[bot]

Fritz.StreamTools/Fritz.StreamTools.csproj

@@ -27,7 +27,7 @@
 		<PackageReference Include="Microsoft.Azure.CognitiveServices.Language.TextAnalytics" Version="3.0.0" />
 		<PackageReference Include="Microsoft.VisualStudio.Web.CodeGeneration.Design" Version="2.2.0" />
 		<PackageReference Include="Octokit" Version="0.32.0" />
-		<PackageReference Include="System.IO.Abstractions" Version="6.0.17" />
+		<PackageReference Include="System.IO.Abstractions" Version="6.0.21" />
 		<PackageReference Include="System.Memory" Version="4.5.3" />
 	</ItemGroup>
 	<ItemGroup>

Test/Test.csproj

@@ -14,7 +14,7 @@
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.2.0" />
 		<PackageReference Include="Microsoft.AspNetCore.App" />
     <PackageReference Include="Moq" Version="4.12.0" />
-    <PackageReference Include="System.IO.Abstractions" Version="6.0.17" />
+    <PackageReference Include="System.IO.Abstractions" Version="6.0.21" />
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.1">
       <PrivateAssets>all</PrivateAssets>