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.

src/common/crypto.rs

@@ -255,7 +255,7 @@ pub async fn get_certificates<'hc>(context: &HuskContext<'hc>, email: &str)
 
     for introducer in &context.introducers {
         log::debug!("consider introducer {:?}",
-            introducer.cert.userids().map(|u| u.userid()).collect::<Vec<_>>());
+            introducer.cert.self_signed_userids().map(|u| u.userid()).collect::<Vec<_>>());
         if introducer.can_introduce(format!("<{}>", email).as_str()) {
             log::debug!("  introducer regex matches!");
             match lookup_certificates(context, email).await {

src/types/introducer.rs

@@ -110,7 +110,7 @@ use std::fmt::{Display, Formatter};
 impl Display for Introducer {
     fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
         // XXX: This is unsafe as ANSI control chars are not handled.
-        let uids = self.cert.userids()
+        let uids = self.cert.self_signed_userids()
             .map(|u| String::from_utf8_lossy(u.userid().value()).to_string())
             .collect::<Vec<String>>()
             .join(", ");