Upgrade to dotnet10 + slnx

[niels9001]

• Upgraded to dotnet10
• Migrated to .slnx
• Updated copilot-instructions.md file with the MS Learn MCP and reference repo's
• Bump year in the About section to 2026

Closes: #1894

Description

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[Copilot]

Pull request overview

This PR upgrades the repo to .NET 10, migrates Visual Studio solutions from .sln to .slnx, and updates documentation/instructions to reflect the new solution format and reference guidance.

Changes:

• Updated target frameworks to net10.0 / net10.0-windows10.0.22621.0.
• Replaced .sln files with .slnx files and updated docs/CI references accordingly.
• Expanded .github/copilot-instructions.md with Microsoft Learn MCP + reference repos.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/WinUIGallery.UITests/WinUIGallery.UITests.csproj	Moves UI test project to net10.0 to align with upgrade.
tests/WinUIGallery.UITests/README.md	Updates automation and VS instructions to use .slnx.
WinUIGallery.slnx	Adds new .slnx solution definition for main solution.
WinUIGallery.sln	Removes old .sln solution file.
WinUIGallery.DesktopWap.slnx	Adds new .slnx solution definition for DesktopWap packaging solution.
WinUIGallery.DesktopWap.sln	Removes old .sln packaging solution file.
README.md	Updates quick start to open .slnx.
Directory.Build.props	Updates Samples TFM moniker to .NET 10 Windows TFM.
.pipelines/azure-pipelines.yml	Updates CI variable to point to .slnx.
.pipelines/WinUI-Gallery-OneBranch-Official.yml	Updates OneBranch pipeline variable to point to .slnx.
.github/copilot-instructions.md	Updates build commands to .slnx and adds doc/reference guidance.

[Copilot]

Since WinUIGallery.slnx is at the repo root (same level as Directory.Build.props in this PR), the relative path ../Directory.Build.props likely points outside the repo. Update this to Directory.Build.props (or the correct in-repo relative path) so Solution Items resolves consistently for contributors.

Suggested change

	<File Path="../Directory.Build.props" />
	<File Path="Directory.Build.props" />

[Copilot]

Same issue as the main solution: ../Directory.Build.props likely points to the parent folder of the repo when the .slnx is at the repo root. Prefer Directory.Build.props (or the correct repo-relative path) so Solution Items loads correctly.

Suggested change

	<File Path="../Directory.Build.props" />
	<File Path="Directory.Build.props" />

[Copilot]

For automation docs, consider publishing the app project directly (e.g., WinUIGallery/WinUIGallery.csproj) rather than the solution file. This avoids coupling the instructions to solution-format support in the installed dotnet/MSBuild version and tends to be more reliable across environments.

Suggested change

	>dotnet.exe publish WinUIGallery.slnx /p:AppxPackageDir=AppxPackages\ /p:platform=x64 /p:PublishProfile=./WinUIGallery/Properties/PublishProfiles/win-x64.pubxml
	>dotnet.exe publish .\WinUIGallery\WinUIGallery.csproj /p:AppxPackageDir=AppxPackages\ /p:platform=x64 /p:PublishProfile=./WinUIGallery/Properties/PublishProfiles/win-x64.pubxml

[Copilot]

The pipeline now builds a .slnx, which requires the agent's MSBuild/Visual Studio tooling to understand the new solution format. To avoid CI breaks from older MSBuild versions on windows-2022, consider explicitly selecting/pinning the Visual Studio/MSBuild version (or adding a validation step) that supports building .slnx.

Suggested change

	vmImage: "windows-2022"
	vmImage: "windows-latest"

[marcelwgn]

/azp run