Revert "Use netstandard3.1 instead of netstandard 2.1"

This reverts commit 23ad8b7.

Author: FreePhoenix888

csharp/Platform.Random/Platform.Random.csproj

@@ -6,7 +6,7 @@
     <AssemblyTitle>Platform.Random</AssemblyTitle>
     <VersionPrefix>0.2.0</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.Random</AssemblyName>
     <PackageId>Platform.Random</PackageId>
     <PackageTags>LinksPlatform;Random;RandomExtensions;RandomHelpers</PackageTags>