[.NET] Add netstandard2.0 as target (#8295)

Co-authored-by: Derek Bailey <derekbailey@google.com>

Author: bjornharrtell

net/FlatBuffers/Google.FlatBuffers.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk" ToolsVersion="12.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
 
   <PropertyGroup>
-    <TargetFrameworks>netstandard2.1;net6.0;net8.0</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;netstandard2.1;net6.0;net8.0</TargetFrameworks>
     <Description>A cross-platform memory efficient serialization library</Description>
     <PackageVersion>24.3.25</PackageVersion>
     <Authors>Google LLC</Authors>