box: prevent demoted leader from being a candidate in the next elections

Currently, the demoted leader sees that nobody has requested a vote in the
newly persisted term (because it has just written it without voting, and
nobody had time to see the new term yet), and hence votes for itself,
becoming the most probable winner of the next elections.

To prevent this from happening, let's forbid the demoted leader to be a
candidate in the next elections using `box_raft_leader_step_off`.

Closes #9855

NO_DOC=<bugfix>

Co-authored-by: Serge Petrenko <sergepetrenko@tarantool.org>