Update target frameworks for Maui and Testing projects

[ChrisPulman]

… when running on Windows

What kind of change does this PR introduce?

fix for #4170

What is the current behavior?

#4170

What is the new behavior?

Refactored target framework definitions in Directory.Build.props to improve cross-platform support for Maui and test projects. Updated ReactiveUI.Testing.csproj to use the correct target frameworks property. These changes ensure more accurate and flexible multi-platform builds.

What might this PR break?

Resolves a previous issue

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

[Copilot]

Pull Request Overview

This PR refactors target framework definitions in MSBuild property files to improve cross-platform support for MAUI and testing projects. The changes address issue #4171 by fixing target framework property usage and ensuring proper platform-specific framework resolution.

• Updates ReactiveUI.Testing.csproj to use the correct target frameworks property
• Refactors MAUI target framework definitions to properly handle Windows-specific targets
• Fixes target framework concatenation to avoid overwriting existing values on Windows

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/ReactiveUI.Testing/ReactiveUI.Testing.csproj	Changed target frameworks property from ReactiveUIFinalModernTargetFrameworks to ReactiveUIFinalTargetFrameworks
src/Directory.Build.props	Refactored MAUI test targets and modern target frameworks to properly handle Windows-specific builds and avoid overwriting existing values

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 34.24%. Comparing base (c339577) to head (7ac638f).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4174      +/-   ##
==========================================
- Coverage   35.25%   34.24%   -1.01%     
==========================================
  Files         140      144       +4     
  Lines        6039     6234     +195     
  Branches      890      926      +36     
==========================================
+ Hits         2129     2135       +6     
- Misses       3746     3932     +186     
- Partials      164      167       +3     

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[Cheesebaron]

These are the TFMs on Reactive.Testing now:

Not sure if the platform specific ones are needed for this package?

[ChrisPulman]

These are now the targets, as per previous versions with net10.0 added

[Cheesebaron]

Thank you so much @ChrisPulman :)