BREAKING: Remove Secure-Join-Fingerprint header from "vc-contact-confirm", "vg-member-added" messages (#3836)

It's a compatibility crutch for old clients (< 1.107.0), they require it in the subject
messages. Currently DC sends Autocrypt key gossips instead, they're better because knowing a key
allows not to only trust the peer, but also encrypt to it. Before it was a problem for other devices
on a joining side going online after a successful Securejoin setup -- they didn't have a joiner's
key to encrypt to it. So, we decided not to complicate the Securejoin with sending keys instead, but
rely on the Autocrypt.

Also there's a PR to the Countermitm doc documenting when gossips in 'vc-request-with-auth' are
needed:

    Bob's own key fingerprint ``Bob_FP``,
    the second challenge ``AUTH`` from step 1 and
    optionally an Autocrypt-Gossip header for Alice's
    Autocrypt key in order for a second device of Bob
    to learn Alice's verified key.

CHANGELOG.md

@@ -6,6 +6,7 @@
 - BREAKING: jsonrpc:
   - `get_chatlist_items_by_entries` now takes only chatids instead of `ChatListEntries`
   - `get_chatlist_entries` now returns `Vec<u32>` of chatids instead of `ChatListEntries`
+- BREAKING: Remove Secure-Join-Fingerprint header from "vc-contact-confirm", "vg-member-added" messages.
 
 
 ## [1.114.0] - 2023-04-24

src/mimefactory.rs

@@ -945,17 +945,6 @@ impl<'a> MimeFactory<'a> {
                             "Secure-Join".to_string(),
                             "vg-member-added".to_string(),
                         ));
-                        // FIXME: Old clients require Secure-Join-Fingerprint header. Remove this
-                        // eventually.
-                        let fingerprint = Peerstate::from_addr(context, email_to_add)
-                            .await?
-                            .context("No peerstate found in db")?
-                            .public_key_fingerprint
-                            .context("No public key fingerprint in db for the member to add")?;
-                        headers.protected.push(Header::new(
-                            "Secure-Join-Fingerprint".into(),
-                            fingerprint.hex(),
-                        ));
                     }
                 }
                 SystemMessage::GroupNameChanged => {

src/securejoin.rs

@@ -465,14 +465,9 @@ pub(crate) async fn handle_securejoin_handshake(
                     info_chat_id(context, contact_id).await?,
                 )
                 .await?;
-                send_alice_handshake_msg(
-                    context,
-                    contact_id,
-                    "vc-contact-confirm",
-                    Some(fingerprint),
-                )
-                .await
-                .context("failed sending vc-contact-confirm message")?;
+                send_alice_handshake_msg(context, contact_id, "vc-contact-confirm", None)
+                    .await
+                    .context("failed sending vc-contact-confirm message")?;
 
                 inviter_progress!(context, contact_id, 1000);
             }
@@ -631,32 +626,6 @@ pub(crate) async fn observe_securejoin_on_other_device(
                 }
                 peerstate.prefer_encrypt = EncryptPreference::Mutual;
                 peerstate.save_to_db(&context.sql).await.unwrap_or_default();
-            } else if let Some(fingerprint) =
-                mime_message.get_header(HeaderDef::SecureJoinFingerprint)
-            {
-                // FIXME: Old versions of DC send this header instead of gossips. Remove this
-                // eventually.
-                let fingerprint = fingerprint.parse()?;
-                if mark_peer_as_verified(
-                    context,
-                    fingerprint,
-                    Contact::load_from_db(context, contact_id)
-                        .await?
-                        .get_addr()
-                        .to_owned(),
-                )
-                .await
-                .is_err()
-                {
-                    could_not_establish_secure_connection(
-                        context,
-                        contact_id,
-                        info_chat_id(context, contact_id).await?,
-                        format!("Fingerprint mismatch on observing {step}.").as_ref(),
-                    )
-                    .await?;
-                    return Ok(HandshakeMessage::Ignore);
-                }
             } else {
                 could_not_establish_secure_connection(
                     context,