Update sk canvas element showcase sample

[DevTKSS]

GitHub Issue: closes #887

This pull request updates the SKCanvasElementShowcase project to target .NET 10.0 across all supported platforms and upgrades the Uno.Sdk version. It also migrates the solution file from the traditional .sln format to the new .slnx format. These changes ensure compatibility with the latest .NET features and tooling.

Target framework and SDK upgrades:

• Updated all target frameworks in SKCanvasElementShowcase.csproj, launch profiles, build scripts, and configuration files from .NET 9.0 to .NET 10.0 for desktop, Android, iOS, Windows, and browser WASM targets. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
• Upgraded Uno.Sdk from version 6.2.29 to 6.4.42 in global.json.

Solution file migration:

• Removed the legacy Visual Studio .sln solution file and replaced it with the new .slnx format for improved project and platform configuration management. [1] [2]

PR Type:

What is the current behavior? 🤔

• uses net9.0
• old sln format solution

What is the new behavior? 🚀

• uses net10.0
• new slnx format solution

PR Checklist ✅

Please check if your PR fulfills the following requirements:

• 📝 Commits must be following the Conventional Commits specification.
• 🧪 Added Runtime tests, UI tests, or a manual test sample for the changes have been added (for bug fixes / features) (if applicable)
• 📚 Docs have been added/updated which fit documentation template (for bug fixes / features)
• 🖼️ Validated PR Screenshots Compare Test Run results.
• ❗ Contains NO breaking changes

Other information ℹ️

• [Docs][SKCanvasElement] Add Troubleshooting to help Consumer and MCP Agents identify and relyably solve problem when happening while implementing this control uno#22214
  • To be updated docs should link to a most current uno.sdk + net target Sample
  • This way the Consumer could compare the .slnx format correct setup for this case as we do need to pay attention to the ARM64 using this guide in the docs