Upgrade distributed caching sample to .NET 10

[sahansera]

What changed

• Upgrade the application from .NET 6 to .NET 10 LTS and pin the SDK.
• Update Microsoft.Extensions.Caching.StackExchangeRedis to 10.0.11.
• Enable nullable reference types and modernize the cache, user service, API client, and controller async APIs.
• Await cache removal before redirecting and add a cache instance prefix.
• Replace the ReqRes users endpoint with JSONPlaceholder.
• Pin the local Redis container to redis:8.8.1-alpine and modernize Docker Compose configuration.
• Update .NET and CodeQL workflows.
• Add a seven-test suite covering serialization, cache behavior, and HTTP handling.

Why

The existing sample targets unsupported .NET 6. Moving to .NET 10 gives readers a supported LTS baseline and fixes stale runtime, dependency, CI, API, and local Redis instructions.

Validation

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• Direct package audit reported no outdated packages

A live Redis integration run was not performed because Docker and Redis were unavailable in the validation environment.