send_ticketdata: update lettre usage

* update Cargo.toml to new lettre crate (v0.10.0-rc.3)
* use TEXT_PLAIN method

Signed-off-by: Ralf Zerres <ralf.zerres@networkx.de>

advotracker_client/src/services/exports/send_ticketdata.rs

@@ -76,16 +76,12 @@ pub fn sendticketdata(email: &Email, lang: &Lang) -> Result<(), Box<dyn Error>>
             MultiPart::alternative() // This is composed of two parts.
                 .singlepart(
                     SinglePart::builder()
-                        .header(header::ContentType(
-                            "text/plain; charset=utf8".parse().unwrap(),
-                        ))
+                        .header(header::ContentType::TEXT_PLAIN)
                         .body(String::from(ascii_body)),
                 )
                 .singlepart(
                     SinglePart::builder()
-                        .header(header::ContentType(
-                            "text/html; charset=utf8".parse().unwrap(),
-                        ))
+                        .header(header::ContentType::TEXT_PLAIN)
                         .body(html.into_string()),
                 ),
         )