[17.0][FIX] hr_course: align schedule tree decoration with Odoo 17 guidelines

[miquelrosell99]

Before this commit, the course schedule tree view colored entire rows based on the state (success for completed, muted for cancelled).

Following Odoo core conventions, workflow state should be rendered as a per-field badge instead of coloring the whole row. This change:

• Removes row-level decoration-success and decoration-muted from the tree.
• Adds widget="badge" to the state field.
• Applies per-state decorations:
  • draft → info
  • waiting_attendees, in_progress, in_validation → warning
  • completed → success
  • cancelled → muted