added dotnet 6.0 framework

fix IFormattable issue

Author: Dominik Nolte

PdfSharpCore/Pdf/PdfInteger.cs

@@ -38,7 +38,7 @@ namespace PdfSharpCore.Pdf
     /// Represents a direct integer value.
     /// </summary>
     [DebuggerDisplay("({Value})")]
-    public sealed class PdfInteger : PdfNumber, IConvertible
+    public sealed class PdfInteger : PdfNumber, IConvertible, IFormattable
     {
         /// <summary>
         /// Initializes a new instance of the <see cref="PdfInteger"/> class.
@@ -175,5 +175,14 @@ uint IConvertible.ToUInt32(IFormatProvider provider)
         }
 
         #endregion
+
+        #region IFormattable Members
+
+        string IFormattable.ToString(string _value, IFormatProvider provider)
+        {
+            return _value;
+        }
+
+        #endregion
     }
 }

PdfSharpCore/PdfSharpCore.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>netstandard1.3;netstandard2.0;netcoreapp3.1;net5.0</TargetFrameworks>
+    <TargetFrameworks>netstandard1.3;netstandard2.0;netcoreapp3.1;net5.0;net6.0</TargetFrameworks>
     <GeneratePackageOnBuild>True</GeneratePackageOnBuild>
     <Authors>Stefan Steiger and Contributors</Authors>
     <Description>PdfSharp for .NET Core