Bump clap to 4.6.6, imap to 3.0.0-alpha.15 and native-tls to 0.2.18. The imap upgrade removes the unsound lexical-core 0.7.6 dependency inherited via nom 5 (RUSTSEC-2023-0086), leaving cargo audit clean. Drop the unused imap-proto dependency and migrate to the imap 3.x ClientBuilder and AppendCmd APIs.