revert to netstandard2.0

Author: AntonioSeric

src/Calendar.Plugin/CalendarPlugin.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="MSBuild.Sdk.Extras/3.0.23">
 
   <PropertyGroup>
-    <TargetFrameworks>netstandard2.1;MonoAndroid10.0;Xamarin.iOS10</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;MonoAndroid10.0;Xamarin.iOS10</TargetFrameworks>
     <Product>$(AssemblyName) ($(TargetFramework))</Product>
     <PackageId>Xamarin.Plugin.Calendar</PackageId>
     <PackageTags>events calendar, calendar, xamarin forms, custom calendar</PackageTags>