chore: upgrade EqDemoWpfNet6 to .NET 8.0

[devin-ai-integration]

Summary

Upgrades the EqDemoWpfNet6 WPF desktop project from .NET 6.0 to .NET 8.0:

• TargetFramework: net6.0-windows → net8.0-windows
• Microsoft.EntityFrameworkCore.SqlServer: 6.0.1 → 8.0.11

Third-party packages (Korzh.EasyQuery.*, Korzh.DbUtils.*, EasyData.Exporters.*) are netstandard2.0-compatible and were left at their current versions. The project compiles with 0 errors and 0 warnings (verified via dotnet build with EnableWindowsTargeting=true on Linux).

Review & Testing Checklist for Human

• Open and build the solution in Visual Studio on Windows — the Linux cross-compilation build confirms package resolution and code compilation, but does not exercise the full WPF toolchain
• Run the application and verify EF Core database operations work — EF Core 6→8 is a major version jump that may introduce runtime behavioral changes (e.g., query translation differences, connection handling) not caught at compile time
• Verify third-party Korzh packages load correctly at runtime — they target netstandard2.0 so should be fine, but worth a smoke test

Notes

• .NET 6 reached end-of-support in November 2024; .NET 8 is the current LTS release
• Build was verified on Linux using EnableWindowsTargeting=true, which allows NuGet restore and compilation but not full WPF runtime testing

Link to Devin session: https://app.devin.ai/sessions/1bf7d6bd37f04ce6b22b4dff4fd86e26
Requested by: @tobydrinkall

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.