Bump openapi-dotnet monorepo to 1.6.28

| datasource | package                   | from   | to     |
| ---------- | ------------------------- | ------ | ------ |
| nuget      | Microsoft.OpenApi         | 1.6.27 | 1.6.28 |
| nuget      | Microsoft.OpenApi.Readers | 1.6.27 | 1.6.28 |


Signed-off-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

src/TodoApp/TodoApp.csproj

@@ -17,7 +17,7 @@
     <PackageReference Include="Microsoft.AspNetCore.OpenApi" Version="9.0.9" />
     <PackageReference Include="Microsoft.EntityFrameworkCore.Sqlite" Version="9.0.9" />
     <PackageReference Include="Microsoft.Extensions.ApiDescription.Server" Version="9.0.9" PrivateAssets="all" />
-    <PackageReference Include="Microsoft.OpenApi" Version="1.6.27" />
+    <PackageReference Include="Microsoft.OpenApi" Version="1.6.28" />
     <PackageReference Include="Microsoft.TypeScript.MSBuild" Version="5.9.3" PrivateAssets="all" />
     <PackageReference Include="NSwag.AspNetCore" Version="14.6.1" />
     <PackageReference Include="Swashbuckle.AspNetCore" Version="9.0.6" />

tests/TodoApp.Tests/TodoApp.Tests.csproj

@@ -12,7 +12,7 @@
     <PackageReference Include="MartinCostello.Logging.XUnit.v3" Version="0.6.0" />
     <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="9.0.9" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.0.0" />
-    <PackageReference Include="Microsoft.OpenApi.Readers" Version="1.6.27" />
+    <PackageReference Include="Microsoft.OpenApi.Readers" Version="1.6.28" />
     <PackageReference Include="Microsoft.Playwright" Version="1.55.0" />
     <PackageReference Include="Verify.XunitV3" Version="30.20.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="3.1.5" />