box: add compat option for replication_sync_timeout default value

Instead of setting the `replication_sync_timeout` default to new value
(0) unconditionally, add a compat option which will control the default
value. The compat option is named "box_cfg_replication_sync_timeout"
and is "old" by default, meaning default `replication_sync_timeout` is
300.

The user has to set it to "new" before the initial box.cfg call for the
default value to change to 0.

Follow-up #5272

NO_DOC=amended an existing doc issue, see https://github.com/tarantool/doc/issues/3295