Added netstandard2.1 again, fixes #74.

Author: SeppPenner

src/Serilog.Sinks.Postgresql.Alternative/Serilog.Sinks.Postgresql.Alternative.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
-        <TargetFrameworks>net6.0;net8.0</TargetFrameworks>
+        <TargetFrameworks>netstandard2.1;net6.0;net8.0</TargetFrameworks>
         <AssemblyName>Serilog.Sinks.Postgresql.Alternative</AssemblyName>
         <RootNamespace>Serilog</RootNamespace>
         <GeneratePackageOnBuild>true</GeneratePackageOnBuild>