Update RegularExpressions package version to 7.3.1 in nuspec file

DEVSIX-6353

Author: Eugene Bochilo

itext7.nuspec

@@ -39,6 +39,7 @@
         <dependency id="System.Xml.XmlDocument" version="4.3.0" />
         <dependency id="System.Security.Cryptography.Csp" version="4.3.0" />
 		<dependency id="Microsoft.DotNet.PlatformAbstractions" version="1.1.0" />
+		<dependency id="System.Text.RegularExpressions" version="4.3.1" />
       </group>
     </dependencies>
   </metadata>