Before this commit, when the task_count is too huge, the number of
open tasks displayed in each kanban card of project returns `NaN` value
the reason is because, for instance, the number is 1026 and using `value`
will format the value and return `NaN` when the soustraction is made
between 2 numbers formatted (task_count - closed_task_count).
This commit introduces a new field called `open_task_count` to get
the number of open tasks and use it in the kanban instead of doing
the soustraction between `task_count` and `closed_task_count` fields
since we cannot really format the result.
closesodoo/odoo#149637
Signed-off-by: Xavier Bol (xbo) <xbo@odoo.com>