Add .NET 10 multitargeting support

[Copilot]

Adds .NET 10.0 as an additional target framework while preserving existing .NET 6 and .NET 8 targets.

Changes

• SDK: Updated global.json to .NET 10.0.100 with latestMinor rollForward
• Projects: Added net10.0-windows to TargetFrameworks in all project files
• CI: Added .NET 10.0.x to GitHub Actions workflow
• WinForms: Fixed WFO1000 designer error by marking AllowUpdates property with [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]

The WFO1000 fix addresses a new .NET 10 requirement where public properties on Form-derived classes must declare their designer serialization behavior:

// PreviewForm.cs
[DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
public bool AllowUpdates { get; set; } = true;

NuGet packages now include assemblies for all three target frameworks.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• aka.ms
  • Triggering command: curl -I -sSL --retry 5 --retry-delay 2 --connect-timeout 15 REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

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This section details on the original issue you should resolve

<issue_title>.NET 10 (Multitargeting)</issue_title>
<issue_description>.NET 10 just came out, update all the projects and test projects. Also make sure to ensure the right SDKs for the build actions.
Use Multitargeting so .NET 10 comes as addition.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes .NET 10 (Multitargeting) #23

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