fix: resolve Snyk Core parse error and ReDoS in test project dependencies

Author: OMpawar-21

Contentstack.Management.Core.Tests/Contentstack.Management.Core.Tests.csproj

@@ -25,6 +25,7 @@
     <PackageReference Include="Moq" Version="4.20.72" />
     <PackageReference Include="Microsoft.AspNetCore.Http" Version="2.3.0" />
     <PackageReference Include="Newtonsoft.Json" Version="13.0.4" />
+    <PackageReference Include="System.Text.RegularExpressions" Version="4.3.1" />
   </ItemGroup>
 
   <ItemGroup>

Contentstack.Management.Core.Unit.Tests/Contentstack.Management.Core.Unit.Tests.csproj

@@ -20,6 +20,7 @@
     <PackageReference Include="AutoFixture.AutoMoq" Version="4.18.1" />
     <PackageReference Include="Moq" Version="4.20.72" />
     <PackageReference Include="Microsoft.AspNetCore.Http" Version="2.3.0" />
+    <PackageReference Include="System.Text.RegularExpressions" Version="4.3.1" />
   </ItemGroup>
 
   <ItemGroup>

Contentstack.Management.Core/contentstack.management.core.csproj

@@ -1,9 +1,13 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
+  <!-- On non-Windows (e.g. macOS/Linux), build only netstandard2.0; .NET Framework (net471/net472) is Windows-only and reference assemblies are not available. -->
+  <PropertyGroup Condition="'$(OS)' == 'Windows_NT'">
+    <TargetFrameworks>netstandard2.0;net471;net472</TargetFrameworks>
+  </PropertyGroup>
+  <PropertyGroup Condition="'$(OS)' != 'Windows_NT'">
+    <TargetFrameworks>netstandard2.0</TargetFrameworks>
+  </PropertyGroup>
   <PropertyGroup>
-    <!-- On non-Windows (e.g. macOS/Linux), build only netstandard2.0; .NET Framework (net471/net472) is Windows-only and reference assemblies are not available. -->
-    <TargetFrameworks Condition="$([MSBuild]::IsOSPlatform('Windows'))">netstandard2.0;net471;net472</TargetFrameworks>
-    <TargetFrameworks Condition="!$([MSBuild]::IsOSPlatform('Windows'))">netstandard2.0</TargetFrameworks>
     <LangVersion>8.0</LangVersion>
     <Nullable>enable</Nullable>
     <Title>Contentstack Management</Title>