[Bug] Don't warn for UWP TargetVersion on Class Libraries

[dotMorten]

Description

When building a class library targeting a versin lower than 17763, but still above the min version of 14393, you get an unnecessary build warning:

Warning Xamarin.Forms recommends TargetPlatformVersion >= 10.0.17763.0 (current project is 17134) C:\Microsoft\Xamarin\NuGet\xamarin.forms\4.5.0.356\build\Xamarin.Forms.targets 199

This is coming from here:

Xamarin.Forms/.nuspec/Xamarin.Forms.targets

Lines 233 to 235 in 4a2a14e

	<Warning
	Text="Xamarin.Forms recommends TargetPlatformVersion &gt;= 10.0.17763.0 (current project is $(MicrosoftUIXamlTargetPlatformCheckValue))"
	Condition="$(MicrosoftUIXamlTargetPlatformCheckValue) &lt; 17763" />

This warning has no use for class libraries and should only apply to application projects. Only thing required here is that we stay within the minversion.

Steps to Reproduce

1. Create a UWP Class library and reference Forms.
2. Target a version below 10.0.17763 but at least 10.0.14393
3. Build.

Expected Behavior

No warning is generated

Actual Behavior

Build Warning about target version being too low.

Basic Information

• Version with issue: 4.5
• Last known good version: N/A
• IDE: VS2019 16.5
• Platform Target Frameworks:
  • UWP: 17134
• Nuget Packages: Xamarin.Forms 4.5.*
• Affected Devices:

Workaround

The targets file doesn't seem to have a variable you can set to skip the version check.
However would be better if the warning was only done for app projects.