Pr/ftp greeting code 220

[ethan-thomason]

fix: use RFC 959 greeting code 220 for FTP greeting

Problem

The FTP handler sends 200 as the initial greeting code when a client connects:

# send 200 + banner -- new client has connected!
self.respond(b"200 " + self.config.banner.encode())

Per [RFC 959 4.2.1](https://datatracker.ietf.org/doc/html/rfc959#section-4.2.1), 220 Service ready for new user is the correct code for a connection greeting. 200 Command okay is a response to a completed command and isn't valid before any command has been issued.

Why it matters

• Client compatibility — some stricter FTP clients reject or warn on a non-220 greeting.
• Fingerprinting — real FTP services on the devices Conpot emulates (e.g. Siemens S7 CPUs) send 220. The 200 greeting is a trivial way to distinguish a Conpot instance from the hardware it's emulating.

Change

One line in conpot/protocols/ftp/ftp_base_handler.py, plus the stale comment above it. No behavior change beyond the response code; the banner text is untouched.

-        # send 200 + banner -- new client has connected!
-        self.respond(b"200 " + self.config.banner.encode())
+        # send 220 + banner -- new client has connected! (RFC 959 greeting code)
+        self.respond(b"220 " + self.config.banner.encode())

Testing

Fresh clone of this branch, uv sync, running the default template:

$ nc localhost 2121
220 FTP server ready.

Previously returned 200 FTP server ready.

black --check . passes — 127 files unchanged.