Bump System.IO.Abstractions from 20.0.15 to 20.0.28 (open-telemetry#3315)

Bumps [System.IO.Abstractions](https://github.com/TestableIO/System.IO.Abstractions) from 20.0.15 to 20.0.28.
- [Release notes](https://github.com/TestableIO/System.IO.Abstractions/releases)
- [Commits](TestableIO/System.IO.Abstractions@v20.0.15...v20.0.28)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

tools/Directory.Packages.props

@@ -4,6 +4,6 @@
     <PackageVersion Include="McMaster.Extensions.CommandLineUtils" Version="4.1.1" />
     <PackageVersion Include="Microsoft.Build" Version="17.6.3" />
     <PackageVersion Include="NuGet.ProjectModel" Version="6.9.1" />
-    <PackageVersion Include="System.IO.Abstractions" Version="20.0.15" />
+    <PackageVersion Include="System.IO.Abstractions" Version="20.0.28" />
   </ItemGroup>
 </Project>