Document the .NET Framework 4.5 SubsystemVersion retargeting change for Windows Forms

[Copilot]

The .NET Framework 4.5 retargeting guide didn't mention that MSBuild changes the default SubsystemVersion from 4.00 to 6.00. That omission leaves a gap for Windows Forms apps that calculate layout from SystemInformation or other GetSystemMetrics-based values and can see size changes after retargeting.

• What changed

  • Added a new Windows Forms retargeting include that documents the SubsystemVersion default change for .NET Framework 4.5.
  • Inserted the new include into docs/framework/migration-guide/retargeting/4.5.x.md under .NET Framework 4.5 > Windows Forms, immediately after the existing EncoderParameter ctor is obsolete entry.
  • Added the required ai-usage frontmatter to the edited article.

• What the new guidance covers

  • The default SubsystemVersion changes from 4.00 to 6.00 when you retarget to .NET Framework 4.5 and build with MSBuild.
  • Windows can report different window metrics under that subsystem version.
  • Windows Forms code that uses xref:System.Windows.Forms.SystemInformation values, such as xref:System.Windows.Forms.SystemInformation.FixedFrameBorderSize, can produce different layout results.
  • A workaround is available when an app must preserve .NET Framework 4 sizing behavior.

• Workaround example

  <PropertyGroup>
    <SubsystemVersion>4.00</SubsystemVersion>
  </PropertyGroup>

---

Internal previews

📄 File	🔗 Preview link
docs/framework/migration-guide/retargeting/4.5.x.md	Retargeting changes for migration to .NET Framework 4.5.x

[Copilot]

Pull request overview

This PR fills a gap in the .NET Framework 4.5 retargeting guide by documenting that MSBuild changes the default SubsystemVersion from 4.00 to 6.00, which can affect Windows Forms layout when apps rely on SystemInformation or other GetSystemMetrics-derived values.

Changes:

• Added a new Windows Forms retargeting include that explains the SubsystemVersion default change and a workaround.
• Inserted the new include into the .NET Framework 4.5 section of the retargeting guide.
• Added ai-usage: ai-assisted frontmatter to the edited article.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
includes/migration-guide/retargeting/winforms/subsystemversion-defaults-to-600.md	New include documenting the SubsystemVersion default change, impact, and workaround.
docs/framework/migration-guide/retargeting/4.5.x.md	Adds ai-usage metadata and includes the new Windows Forms retargeting entry.

[gewarren]

I guess it doesn't need a TOC entry?