[FIX] project: make _get_subtasks_recursively actually recursive

Previous commit (https://github.com/odoo/odoo/pull/116570

) renamed
this method _get_subtasks_recursively.
But instead of calling itself in the return statment, the
method _get_all_subtasks was called resulting in a non-recursive
function and to potential undesired behavior as _get_all_subtasks calls
_get_subtask_ids_per_task_id that calls _get_subtasks_recursively in
some cases.

With this commit, we just call _get_subtasks_recursively on the children
to make it actually recursive, like it was in the first place.

closes odoo/odoo#122331

Signed-off-by: Xavier Bol (xbo) <xbo@odoo.com>