chore: Use Convert.ToHexString (#2322)

* Use Convert.ToHexString

Use `Convert.ToHexString()` when targeting .NET 8 instead of using a `StringBuilder`.

* Update src/Microsoft.OpenApi/Models/OpenApiDocument.cs

---------

Co-authored-by: Vincent Biret <[email protected]>

Author: martincostello

src/Microsoft.OpenApi/Models/OpenApiDocument.cs

@@ -544,13 +544,17 @@ private static string ConvertByteArrayToString(byte[] hash)
         {
             // Build the final string by converting each byte
             // into hex and appending it to a StringBuilder
+#if NET5_0_OR_GREATER
+            return Convert.ToHexString(hash);
+#else
             var sb = new StringBuilder();
             for (var i = 0; i < hash.Length; i++)
             {
                 sb.Append(hash[i].ToString("X2"));
             }
 
             return sb.ToString();
+#endif
         }
 
         /// <summary>