Update solution files

[imalcolm1]

Description

Now that all projects have been moved to .NET 10, all users should be using VS 2026, which supports slnx files. This PR migrates from sln to slnx. Also took the chance to move the CatalogGenerator project out of the WinUI folder and add Directory.Build.props to the solution (finally).

Type of change

• Other enhancement

Platforms tested on

• Visual Studio 2026

Checklist

• Self-review of changes
• All changes work as expected on all affected platforms
• There are no warnings related to changes
• Code is commented and follows .NET conventions and standards
• Codemaid and XAML styler extensions have been run on every changed file
• No unrelated changes have been made to any other code or project files
• Screenshots are correct size and display in description tab (800 x 600 on Windows, MAUI mobile platforms use the MAUI Windows screenshot)

[praveenaak]

Awesome! Thank you for updating this.

Super minor nit for the README:

arcgis-maps-sdk-dotnet-samples/README.md

Lines 37 to 43 in 13454ae

	- All: `src\ArcGIS.Viewers.All.sln`
	If you are only interested in one platform, you can open a platform-specific solutions:
	- [WPF .NET](src/WPF/readme.md): `src\WPF\WPF.Viewer.Net.sln`
	- [.NET MAUI](src/MAUI/readme.md): `src\MAUI\ArcGIS.Samples.Maui.sln`
	- [WinUI](src/WinUI/readme.md): `src\WinUI\ArcGIS.WinUI.Viewer.sln`

-- All: `src\ArcGIS.Viewers.All.sln`
+- All: `src\ArcGIS.AllSampleViewers.slnx`

[imalcolm1]

Super minor nit for the README

Ah good catch, thanks!