NetCore - Update System.Drawing.Common to 4.7.3

- Stop the dependency bot from complaining. 4.7.0 is only vulnerable on non-windows systems
  so it wasn't actually a problem as CefSharp only runs on windows

Author: amaitland

NuGet/PackageReference/CefSharp.OffScreen.NETCore.nuspec

@@ -14,7 +14,7 @@
     <dependencies>
       <group targetFramework=".NETCoreApp3.1">
         <dependency id="CefSharp.Common.NETCore" version="[$version$]" include="Runtime,Compile,Build,Native" />
-		<dependency id="System.Drawing.Common" version="4.7.0" />
+		<dependency id="System.Drawing.Common" version="4.7.3" />
       </group>
     </dependencies>
     <releaseNotes>