Update target frameworks to netstandard2.0 + net461

Author: abnegate

templates/dotnet/src/Appwrite/Appwrite.csproj.twig

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>net47;net48;netstandard1.3;netstandard2.0;netstandard2.1;netcoreapp3.1</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;net461</TargetFrameworks>
     <PackageId>{{spec.title}}</PackageId>
     <Version>{{sdk.version}}</Version>
     <Authors>{{spec.contactName}}</Authors>