switch to old syntax for build server

src/UglyToad.PdfPig.Tokenization/NumericTokenizer.cs

@@ -8,7 +8,7 @@
 
     internal class NumericTokenizer : ITokenizer
     {
-        private readonly StringBuilder stringBuilder = new();
+        private readonly StringBuilder stringBuilder = new StringBuilder();
 
         private const byte Zero = 48;
         private const byte Nine = 57;

src/UglyToad.PdfPig.Tokenization/PlainTokenizer.cs

@@ -6,7 +6,7 @@
 
     internal class PlainTokenizer : ITokenizer
     {
-        private readonly StringBuilder stringBuilder = new();
+        private readonly StringBuilder stringBuilder = new StringBuilder();
 
         public bool ReadsNextByte { get; } = true;
 

src/UglyToad.PdfPig.Tokenization/StringTokenizer.cs

@@ -6,7 +6,7 @@
 
     internal class StringTokenizer : ITokenizer
     {
-        private readonly StringBuilder stringBuilder = new();
+        private readonly StringBuilder stringBuilder = new StringBuilder();
         public bool ReadsNextByte { get; } = false;
 
         public bool TryTokenize(byte currentByte, IInputBytes inputBytes, out IToken token)