Update RegularExpressions package version to 7.3.1

DEVSIX-6353

Author: Eugene Bochilo

itext/itext.io/itext.io.netstandard.csproj

@@ -85,6 +85,7 @@
     <PackageReference Include="System.Text.Encoding.CodePages" Version="4.3.0" />
 	  <PackageReference Include="Microsoft.DotNet.PlatformAbstractions" Version="1.1.0" />
 	  <PackageReference Include="System.Xml.XmlDocument" Version="4.3.0" />
+	  <PackageReference Include="System.Text.RegularExpressions" Version="4.3.1" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\itext.commons\itext.commons.netstandard.csproj" />

itext/itext.kernel/itext.kernel.netstandard.csproj

@@ -31,6 +31,7 @@
     <PackageReference Include="System.Threading.Thread" Version="4.3.0" />
     <PackageReference Include="System.Threading.ThreadPool" Version="4.3.0" />
     <PackageReference Include="System.Xml.XmlDocument" Version="4.3.0" />
+    <PackageReference Include="System.Text.RegularExpressions" Version="4.3.1" />
   </ItemGroup>
   <PropertyGroup>
     <NoWarn>1701;1702;1591;1570;1572;1573;1574;1580;1584;1658</NoWarn>

itext/itext.pdftest/itext.pdftest.netstandard.csproj

@@ -23,6 +23,7 @@
     <PackageReference Include="System.Collections.Concurrent" Version="4.3.0" />
     <PackageReference Include="System.Reflection.Extensions" Version="4.3.0" />
     <PackageReference Include="System.Xml.XmlDocument" Version="4.3.0" />
+    <PackageReference Include="System.Text.RegularExpressions" Version="4.3.1" />
     <!--    
      Explicitly add System.Net.Http v4.3.4 dependency.
      Without this explicit dependency Rider IDE somehow cannot properly resolve it, which ends up in FileNotFound exception