Retarget UWP to 18382\1903

[inforithmics]

Retarget to 18382\1903

Description of Bug

Retargets to Windows 1903 so that Projects that target 1909 can still be supported. (End of Life of 1909 is May Next year)

Issues Fixed

• Fixes [Bug] Popup on UWP throws an exception when accessed from the nightly nuget #889

Behavioral Changes

Projects that Target 1903 / 1909 work now.

PR Checklist

• Has a linked Issue, and the Issue has been approved
• Has tests (if omitted, state reason in description)
  Only retargets Xamarin Community Toolkit no new functionality is introduced
• Has samples (if omitted, state reason in description)
  Only retargets Xamarin Community Toolkit no new functionality is introduced
• Rebased on top of main at time of PR
• Changes adhere to coding standard

• Updated documentation

[YZahringer]

I think that the support of Windows 1809 Build 17763 removed in #653 (comment) should be restored.
Windows 1809 is the latest LTSC version and the EOL is January 9, 2029.

[bijington]

@inforithmics thank you for this change!