Bump System.IO.Abstractions.TestingHelpers from 20.0.4 to 20.0.15

Bumps [System.IO.Abstractions.TestingHelpers](https://github.com/TestableIO/System.IO.Abstractions) from 20.0.4 to 20.0.15.
- [Release notes](https://github.com/TestableIO/System.IO.Abstractions/releases)
- [Commits](TestableIO/System.IO.Abstractions@v20.0.4...v20.0.15)

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

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

src/BicepButton/BicepButton.csproj

@@ -11,7 +11,7 @@
     <PackageReference Include="Microsoft.Azure.Functions.Worker.Extensions.Http" Version="3.1.0" />
     <PackageReference Include="Microsoft.Azure.Functions.Worker.Sdk" Version="1.16.4" />
     <PackageReference Include="Azure.Bicep.Core" Version="0.24.24" />
-    <PackageReference Include="System.IO.Abstractions.TestingHelpers" Version="20.0.4" />
+    <PackageReference Include="System.IO.Abstractions.TestingHelpers" Version="20.0.15" />
   </ItemGroup>
   <ItemGroup>
     <None Update="host.json">