Enable anonymous OpenPGP key IDs

[link2xt]

This feature was disabled due to interoperability problems: https://github.com/chatmail/core/issues/7378

PR that disabled anonymous OpenPGP recipients: https://github.com/chatmail/core/pull/7383

We should revert https://github.com/chatmail/core/pull/7383, rough plan is to do this beginning of 2026.

There should be an announcement in the device message of the release that says there was a breaking change and that in case of problems and users receiving messages that their recipients cannot decrypt, they need to tell them to upgrade. We may also enable it only for group messages at first so 1:1 chats keep compatible for some more time.

I also posted this forum topic to remind everyone to upgrade: https://support.delta.chat/t/psa-known-interoperability-problem-in-chatmail-core-1-160-0/4284