fix: Removes support for v4 Client and old gump packet. (#1739)

### Summary
* Removes old gump packet support
* Removes support for v4 clients
* Removes `Unpack` flag and assumes it is always true.
* Removes StringToBuffer since this is built into .NET now.

> [!Note]
> View the file changes with white space off: https://github.com/modernuo/ModernUO/pull/1739/files?diff=split&w=1

Projects/Server.Tests/Helpers/PacketWriter.cs

@@ -147,6 +147,14 @@ namespace Server.Network
             UnderlyingStream.Write(buffer, offset, size);
         }
 
+        /// <summary>
+        ///     Writes a sequence of bytes to the underlying stream
+        /// </summary>
+        public void Write(ReadOnlySpan<byte> buffer)
+        {
+            UnderlyingStream.Write(buffer);
+        }
+
         /// <summary>
         ///     Writes a fixed-length ASCII-encoded string value to the underlying stream. To fit (size), the string content is either
         ///     truncated or padded with null characters.