Update samples to use .net 8 (At least)

[yeelam-gordon]

Description

Given .net 6 already EOL, at least we should update all project to .net 8.

This pull request updates multiple C# sample projects to modernize their .NET framework versions and standardize runtime identifiers. The changes improve compatibility with newer .NET features and align runtime naming conventions across all samples.

.NET Framework and Runtime Modernization:

• Updated the TargetFramework for all sample .csproj files from net6.0-windows10.0.19041.0 to net8.0-windows10.0.19041.0, ensuring support for the latest .NET features and APIs. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
• Standardized RuntimeIdentifiers by replacing win10-* identifiers with win-* (e.g., win10-x64 → win-x64) across all projects for consistency and future-proofing. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

These updates help keep the sample projects current and compatible with the latest development tools.

Target Release

Please specify which release this PR should align with 1.8

Checklist

Note that /azp run currently isn't working for this repo.

• Samples build and run using the Visual Studio versions listed in the Windows development docs.
• Samples build and run on all supported platforms (x64, x86, ARM64) and configurations (Debug, Release).
• Samples set the minimum supported OS version to Windows 10 version 1809.
• Samples build clean with no warnings or errors.
• [For new samples]: Samples have completed the sample guidelines checklist and follow standardization/naming guidelines.
• Microsoft employees only: you can validate your changes by following the instructions here: https://www.osgwiki.com/wiki/WindowsAppSDK-Samples

[Copilot]

Pull Request Overview

This pull request modernizes the Windows App SDK sample projects by updating them from .NET 6 (which has reached end-of-life) to .NET 8, and standardizes runtime identifiers across all projects. The changes ensure compatibility with the latest .NET features and align with current naming conventions.

Key changes:

• Updated all C# project files from net6.0-windows10.0.* to net8.0-windows10.0.* target frameworks
• Standardized RuntimeIdentifiers from win10-x86/x64/arm64 to win-x86/x64/arm64 format
• Enhanced the build script with conditional NuGet restore logic and parallel build support

Reviewed Changes

Copilot reviewed 44 out of 44 changed files in this pull request and generated 4 comments.

File	Description
build.ps1	Added conditional NuGet restore for packages.config-based projects, enabled parallel builds with /restore flag, and refactored elapsed time formatting
Samples/nuget.config	Added commented-out nuget.org source for local development reference
Multiple .csproj files (31 files)	Updated TargetFramework from net6.0 to net8.0 and standardized RuntimeIdentifiers from win10-* to win-* format

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