[IMP] Set birthdate to required on sub request

The birthdate is necessary for some later cooperator actions, so it must be set as required.

cooperator/models/subscription_request.py

@@ -211,6 +211,7 @@ def _compute_subscription_amount(self):
     birthdate = fields.Date(
         string="Date of birth",
         readonly=True,
+        required=True,
         states={"draft": [("readonly", False)]},
     )
     gender = fields.Selection(