Upgrade in-memory caching sample to .NET 10

[sahansera]

What changed

• Target only .NET 10 LTS and pin the SDK with global.json.
• Update GitHub Actions to restore, build, and test the .NET 10 Release configuration.
• Replace the ReqRes users endpoint, which now requires an API key, with JSONPlaceholder.
• Align the user model, views, tests, and README with the updated sample.
• Remove the unused placeholder test.

Why

.NET 8 and .NET 9 reach end of support in November 2026. Keeping the default branch on .NET 10 avoids presenting readers with soon-to-be unsupported targets and keeps the tutorial sample runnable without third-party API credentials.

Validation

• dotnet restore
• dotnet build --no-restore --configuration Release
• dotnet test --no-build --configuration Release --verbosity normal
• 10 tests passed
• Build completed with 0 warnings and 0 errors
• Direct package audit reported no outdated packages