Update to new core, which changes invite links to new format

[Simon-Laux]

Since chatmail/core#5467, invite links have the i.delta.chat format which has the benefit of also fallback that explains to install delta chat if people without delta chat open it.

It has the disadvantage that it doesn't work if delta chat isn't installed and i.delta.chat is not reachable. - so guides should still mention how to get the app and not only rely on the new code.