Update dependency System.IO.Abstractions to v21

Author: renovate[bot]

System.IO.Abstractions.Analyzers.Tests/System.IO.Abstractions.Analyzers.Tests.csproj

@@ -8,7 +8,7 @@
 
 	<ItemGroup>
 		<PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.6.3" />
-		<PackageReference Include="System.IO.Abstractions" Version="19.1.14" />
+		<PackageReference Include="System.IO.Abstractions" Version="21.2.12" />
 		<PackageReference Include="xunit" Version="2.6.2" />
 		<PackageReference Include="xunit.runner.visualstudio" Version="2.5.4">
 		  <PrivateAssets>all</PrivateAssets>