`Chat-Version:` header seems to do more harm than good meanwhile

the `Chat-Version:` header may do more harm than good nowadays.

e.g. [mailcow seems to move messages _by default_ with this header unconditionally to the `DeltaChat` folder](https://support.delta.chat/t/troubleshooting-classic-email-as-relay-deltachat-folder-moving-by-mailcow/5633/17?u=r10s), resulting in messages just being missed on the recent Delta Chat updates. maybe there are more such automations.

if we do not use the `DeltaChat` folder any longer internally, and the main purpose of external usage of this header was to move messages server-side around, it seems logical to drop the header as well.

only reason to maintain it, is if we still need it internally, i do not have deep knowledge over that, but reading https://github.com/chatmail/core/pull/7932 it does not look like it, the PR even suggests to "moving Chat-Version must happen at least one release after the PR here is released"

for existing filters, well, the server obviously anyways need adaption and can then be adapted for filtering eg. by "message encrypted" or "subject = [...]". both a bit hands on, but good enough regarding the fact that we do no longer support "shared usecase" at all