This commit removes the tooltip of the `fold` field because
the second part is no longer true and the first part does not
provide any added value.
It also makes `project_ids` field required in the form view of
Task Stage model (`project.task.type`) to force the user to link
his new stage to a project. Otherwise, the stage won't ever be
visible anywhere in the interface. Indeed, stages not holding
tasks are hidden in cross-projects views.
task-3251621
closesodoo/odoo#121154
Signed-off-by: Xavier Bol (xbo) <xbo@odoo.com>