Bump the system-io group with 2 updates

Bumps the system-io group with 2 updates: [System.IO.Abstractions](https://github.com/TestableIO/System.IO.Abstractions) and [System.IO.Abstractions.TestingHelpers](https://github.com/TestableIO/System.IO.Abstractions).


Updates `System.IO.Abstractions` from 19.2.69 to 20.0.15
- [Release notes](https://github.com/TestableIO/System.IO.Abstractions/releases)
- [Commits](TestableIO/System.IO.Abstractions@v19.2.69...v20.0.15)

Updates `System.IO.Abstractions.TestingHelpers` from 19.2.69 to 20.0.15
- [Release notes](https://github.com/TestableIO/System.IO.Abstractions/releases)
- [Commits](TestableIO/System.IO.Abstractions@v19.2.69...v20.0.15)

---
updated-dependencies:
- dependency-name: System.IO.Abstractions
  dependency-type: direct:production
  update-type: version-update:semver-major
  dependency-group: system-io
- dependency-name: System.IO.Abstractions.TestingHelpers
  dependency-type: direct:production
  update-type: version-update:semver-major
  dependency-group: system-io
...

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

Author: dependabot[bot]

test/Tests.csproj

@@ -38,7 +38,7 @@
     </PackageReference>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.7.1" />
     <PackageReference Include="Shouldly" Version="4.2.1" />
-    <PackageReference Include="System.IO.Abstractions.TestingHelpers" Version="19.2.69" />
+    <PackageReference Include="System.IO.Abstractions.TestingHelpers" Version="20.0.15" />
     <PackageReference Include="Verify.Xunit" Version="20.8.1" />
     <PackageReference Include="xunit" Version="2.5.0" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.5.0">