Use stackalloc in NumberFormatterBase (#67003)

Co-authored-by: NTrayan Zapryanov <Traian.Zaprianov@docuware.com>

src/libraries/System.Private.Xml/src/System/Xml/Xsl/Runtime/NumberFormatter.cs

@@ -82,7 +82,7 @@ public static void ConvertToAlphabetic(StringBuilder sb, double val, char firstC
             Debug.Assert(1 <= val && val <= MaxAlphabeticValue);
             Debug.Assert(Math.Pow(totalChars, MaxAlphabeticLength) >= MaxAlphabeticValue);
 
-            char[] letters = new char[MaxAlphabeticLength];
+            Span<char> letters = stackalloc char[MaxAlphabeticLength];
             int idx = MaxAlphabeticLength;
             int number = (int)val;
 
@@ -93,7 +93,7 @@ public static void ConvertToAlphabetic(StringBuilder sb, double val, char firstC
                 number = quot;
             }
             letters[--idx] = (char)(firstChar + --number);
-            sb.Append(letters, idx, MaxAlphabeticLength - idx);
+            sb.Append(letters.Slice(idx, MaxAlphabeticLength - idx));
         }
 
         protected const int MaxRomanValue = 32767;