nuspec: update package dependencies

* require Xamarin.Forms >= 5.0
  (we currently use 5.0.0.2545)
* add dependency on Xamarin.Essentials
  (anything above 1.3 should be fine; we currently use 1.7.3)

Author: janusw

Source/OxyPlot.Xamarin.Forms.nuspec

@@ -16,23 +16,28 @@
     <dependencies>
       <group targetFramework="Xamarin.iOS10">
         <dependency id="OxyPlot.Core" version="2.0.0"/>
-        <dependency id="Xamarin.Forms" version="3.0.0.482510" />
+        <dependency id="Xamarin.Forms" version="5.0" />
+        <dependency id="Xamarin.Essentials" version="1.3" />
       </group>
       <group targetFramework="MonoAndroid10">
         <dependency id="OxyPlot.Core" version="2.0.0"/>
-        <dependency id="Xamarin.Forms" version="3.0.0.482510" />
+        <dependency id="Xamarin.Forms" version="5.0" />
+        <dependency id="Xamarin.Essentials" version="1.3" />
       </group>
       <group targetFramework="uap10.0">
         <dependency id="OxyPlot.Core" version="2.0.0"/>
-        <dependency id="Xamarin.Forms" version="3.0.0.482510" />
+        <dependency id="Xamarin.Forms" version="5.0" />
+        <dependency id="Xamarin.Essentials" version="1.3" />
       </group>
       <group targetFramework="Xamarin.Mac20">
         <dependency id="OxyPlot.Core" version="2.0.0"/>
-        <dependency id="Xamarin.Forms" version="3.0.0.482510" />
+        <dependency id="Xamarin.Forms" version="5.0" />
+        <dependency id="Xamarin.Essentials" version="1.3" />
       </group>
       <group targetFramework="netstandard2.0">
         <dependency id="OxyPlot.Core" version="2.0.0"/>
-        <dependency id="Xamarin.Forms" version="3.0.0.482510" />
+        <dependency id="Xamarin.Forms" version="5.0" />
+        <dependency id="Xamarin.Essentials" version="1.3" />
       </group>
     </dependencies>
   </metadata>