Remove mvbox_move and only_fetch_mvbox

[link2xt]

only_fetch_mvbox option is moved to per-relay configuration parameter imap_folder, similar to IMAP port.

First commit closes #7655

Second commit closes #7648

Options mvbox_move and only_fetch_mvbox are removed.

mvbox_move was deprecated already and there is a device message telling users to disable it.

only_fetch_mvbox is replaced with a per-transport "IMAP folder" option, similar to "IMAP user". Selecting the correct IMAP folder is essentially a part of the login process. By default "INBOX" is selected, but to support existing setups, this can be set to "DeltaChat" or "INBOX.DeltaChat" or similar configuration.

In the core crate, EnteredLoginParam, now has a nested "imap.folder" field of string type, similar to "imap.port" or "imap.user". It is non-optional, but can be an empty string which means user has not entered anything. In this case during configuration "INBOX" folder will be assumed. This is the type stored in the entered_param column of the transports table.

JSON-RPC type EnteredLoginParam, which is a flat dictionary, now includes optional imap_folder value.
If it is not set, watched folder is "INBOX", otherwise the one set here.
It should not be an empty string.

[Hocuri]

Is this ready for review? Because #7655 says "one release after #7654 is merged". #7654 got into v2.36.0, which is only being released now with the current releases, and we may need to do a bugfix release.

[link2xt]

It is ready for review, but I have not tested a migration yet on my test account that uses "only fetch mvbox" and not going to merge it soon even after review so it does not get into bugfix releases. It bumps dcbackup version because downgrade may result in accidentally reading inbox instead of the other folder, so it is aimed for the next larger release.