Upgrade dependencies

- Upgrade Sequoia PGP to 2.4.0
- Remove deprecated `sequoia_wot::CertSynopsis::userids()` and replace
  with `self_signed_userids()`.
- Add `lettre` as dependency for its SMTP functionality.

Cargo.toml

@@ -17,6 +17,7 @@ bytes = "1.11.1"
 clap = { version = "4", features = ["derive"] }
 fastrand = "2.4.1"
 indymilter = "0.3.0"
+lettre = "0.11.22"
 log = "0.4.30"
 log4rs = "1.3.0"
 sequoia-cert-store = "0.7.3"
@@ -41,3 +42,7 @@ daemonix = "0.1.0"
 anyhow = "1.0.100"
 clap = { version = "4", features = ["derive"] }
 clap_complete = "4"
+
+[profile.release]
+lto = true
+codegen-units = 1