Fix: Ensure SupportedOSPlatformVersion is respected by installer when TargetPlatformMinVersion is not set

[Copilot]

Issue

When a .NET project sets both TargetFramework (e.g., net8.0-windows10.0.22621.0) and SupportedOSPlatformVersion (e.g., 10.0.19041.0), the installer wouldn't allow installation on Windows versions older than the version specified in TargetFramework, ignoring the SupportedOSPlatformVersion setting.

This happens because:

1. The TargetPlatformMinVersion property is used to determine the minimum OS version in the AppxManifest.xml
2. For .NET projects (especially WinUI apps), TargetPlatformMinVersion is often not set explicitly
3. When TargetPlatformMinVersion is not set, the build system doesn't automatically use SupportedOSPlatformVersion as a fallback

Solution

This PR updates the MSBuild targets to set TargetPlatformMinVersion based on SupportedOSPlatformVersion when TargetPlatformMinVersion is not explicitly defined. This ensures the correct minimum OS version is written to the AppxManifest.xml file.

Changes:

• Added an MSBuild target to both .NET and C++ project targets files that sets TargetPlatformMinVersion from SupportedOSPlatformVersion when appropriate
• Updated common targets to ensure this behavior is consistent
• Added documentation in the FAQ explaining this behavior

With these changes, developers can correctly specify their minimum supported OS version through SupportedOSPlatformVersion and have it respected by the installer, while still targeting newer OS features through TargetFramework.

Example project configuration that will now work correctly:

<TargetFramework>net8.0-windows10.0.22621.0</TargetFramework>
<SupportedOSPlatformVersion>10.0.19041.0</SupportedOSPlatformVersion>

Fixes #7.

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