add if-defs for libs

Author: determ1ne

src/libraries/System.Security.Cryptography.Pkcs/src/Internal/Cryptography/PkcsHelpers.cs

@@ -364,11 +364,16 @@ public static string ToSerialString(this byte[] serialBytes)
             return ToUpperHexString(serialBytes);
         }
 
-#if NETCOREAPP || NETSTANDARD2_1
+#if NET5_0_OR_GREATER
         private static string ToUpperHexString(ReadOnlySpan<byte> ba)
         {
             return Convert.ToHexString(ba);
         }
+#elif NETCOREAPP || NETSTANDARD2_1
+        private static string ToUpperHexString(ReadOnlySpan<byte> ba)
+        {
+            return HexConverter.ToString(ba, HexConverter.Casing.Upper);
+        }
 #else
         private static string ToUpperHexString(ReadOnlySpan<byte> ba)
         {

src/libraries/System.Security.Cryptography.Xml/src/System/Security/Cryptography/Xml/SignedXmlDebugLog.cs

@@ -197,7 +197,11 @@ private static string FormatBytes(byte[]? bytes)
             if (bytes == null)
                 return NullString;
 
+#if NET9_0_OR_GREATER
             return Convert.ToHexStringLower(bytes);
+#else
+            return HexConverter.ToString(bytes, HexConverter.Casing.Lower);
+#endif
         }
 
         /// <summary>

src/libraries/System.Security.Cryptography.Xml/src/System/Security/Cryptography/Xml/Utils.cs

@@ -720,10 +720,17 @@ internal static X509Certificate2Collection BuildBagOfCerts(KeyInfoX509Data keyIn
             return collection;
         }
 
+#if NET5_0_OR_GREATER
         internal static string EncodeHexString(byte[] sArray)
         {
             return Convert.ToHexString(sArray);
         }
+#else
+        internal static string EncodeHexString(byte[] sArray)
+        {
+            return HexConverter.ToString(sArray);
+        }
+#endif
 
         internal static byte[] DecodeHexString(string s)
         {