To achieve that, we bypass the read-only check for the _truncate system
space in box_process1() and perform it in the on_replace system trigger
instead, when we know which space is truncated.

Note, we have to move the check for insertion of a new record into the
_truncate system space before the read-only check in the on_replace
trigger callback; this is needed for initial recovery with a non-empty
_truncate space to work. While we are at it, let's use recovery_state to
make the check explicit.

Closes #5616

@TarantoolBot document
Title: Mention that temp and local spaces can be truncated in ro mode

DML operations on temporary and local spaces can be performed even if
the instance is in the read-only mode, but DDL operations (such as
`alter`) are forbidden in this case[^1]. Technically, `truncate` is
a DDL operation so initially it was forbidden as well. However, it
should be safe to perform this operation on a temporary or local space
because logically it only modifies the data stored in the space (like
DML) and it isn't replicated (see tarantool/tarantool#4263). So starting
from Tarantool 2.11.1 we allow users to truncate temporary spaces in the
read-only mode.

[^1]: https://www.tarantool.io/en/doc/latest/concepts/replication/repl_architecture/#replication-local

(cherry picked from commit 054526ac)