Bump System.IO.Abstractions.TestingHelpers from 21.2.8 to 21.3.1

Bumps [System.IO.Abstractions.TestingHelpers](https://github.com/TestableIO/System.IO.Abstractions) from 21.2.8 to 21.3.1.
- [Release notes](https://github.com/TestableIO/System.IO.Abstractions/releases)
- [Commits](TestableIO/System.IO.Abstractions@v21.2.8...v21.3.1)

---
updated-dependencies:
- dependency-name: System.IO.Abstractions.TestingHelpers
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

Directory.Packages.props

@@ -31,6 +31,6 @@
     <PackageVersion Include="Moq" Version="4.20.72" />
     <PackageVersion Include="NUnit" Version="4.3.2" />
     <PackageVersion Include="NUnit3TestAdapter" Version="4.6.0" />
-    <PackageVersion Include="System.IO.Abstractions.TestingHelpers" Version="21.2.8" />
+    <PackageVersion Include="System.IO.Abstractions.TestingHelpers" Version="21.3.1" />
   </ItemGroup>
 </Project>