fix: set backward verification when observing vc-contact-confirm or `vg-member-added`

[link2xt]

Documentation comment says forward and backward verification is set, but the code was not doing it. vc-contact-confirm and vg-member-added messages indicate that other device finished securejoin protocol so we know Bob has our key marked as verified.

Fixes #5929

[link2xt]

There are also unrelated bugs with synchronization.

Second Alice's device logs src/sync.rs:303: Ignoring token for nonexistent/deleted group 'IiHMYbcrF7S'. because by the time QR code is generated group is not promoted. Maybe it is fine, only one device processing join requests is not critical. Second device actually does not have group name and avatar so it cannot properly process join requests.

As the QR code is unknown to Alice's second device, when it receives vg-request-with-auth it logs src/securejoin.rs:697: StockMessage::ContactNotVerified posted to 1:1 chat (Auth invalid.). This actually happens even before receiving a sync message because in the test Bob scans Alice's QR code faster than Alice sends a sync message. Posted message "Cannot establish guaranteed end-to-end encryption with ..." is annoying and should not be posted on Alice's side just because it sees message from Bob with unknown AUTH code. Probably should just be ignored. It is Bob who wants to establish contact, Alice should not receive non-actionable error messages.

I opened a separate issue for this: https://github.com/deltachat/deltachat-core-rust/issues/5932

[link2xt]

Based it on stable. Unfortunately, will have to make 1.142.11.