Revert "Use netstandard3.1 instead of netstandard 2.1"

This reverts commit afc7158.

Author: FreePhoenix888

csharp/Platform.RegularExpressions.Transformer.CSharpToPython/Platform.RegularExpressions.Transformer.CSharpToPython.csproj

@@ -6,7 +6,7 @@
     <AssemblyTitle>Platform.RegularExpressions.Transformer.CSharpToPython</AssemblyTitle>
     <VersionPrefix>0.0.1</VersionPrefix>
     <Authors>Konstantin Diachenko</Authors>
-    <TargetFrameworks>net472;netstandard2.0;netstandard3.1;net5;net6</TargetFrameworks>
+    <TargetFrameworks>net472;netstandard2.0;netstandard2.1;net5;net6</TargetFrameworks>
     <AssemblyName>Platform.RegularExpressions.Transformer.CSharpToPython</AssemblyName>
     <PackageId>Platform.RegularExpressions.Transformer.CSharpToPython</PackageId>
     <PackageTags>LinksPlatform;RegularExpressions.Transformer.CSharpToPython;CSharpToPythonTransformer</PackageTags>