Gossip.join() API is dangerous

[link2xt]

If you want to break compatibility at some point, would be nice to deprecate https://docs.rs/iroh-gossip/0.27.0/iroh_gossip/net/struct.Gossip.html#method.join

It calls .joined().await internally which I would rather call explicitly. Otherwise it is too easy to block the whole app or forget the timeout: https://github.com/deltachat/deltachat-core-rust/pull/6110