Before this commit, the milestones are only linked to a project without
any link to the tasks of the same project. However, the tasks could be
considered as steps to reach a project milestone.
This commit adds a link between the `project.task` and
`project.milestone` models. This link is added in the `project.task`
model with the Many2One field called `milestone_id`. With this field,
the user will be able to link a milestone to a task.
task-2829542
This commit improves the project updates panel and the data reported
in the update description.
This commit prepares the removal of the project overview feature.
PR : #72736
See odoo/upgrade#2706
task-2545084
Xavier BOL (xbo)