[IMP] Set birthdate to required on sub request

The birthdate is necessary for some later cooperator actions, so it
must be set as required.
It is set in the view and not in python because it must be compatible
with previous data in birthdate might not have been set.

cooperator/views/subscription_request_view.xml

@@ -129,7 +129,7 @@ SPDX-License-Identifier: AGPL-3.0-or-later
                             />
                             <field name="firstname" />
                             <field name="lastname" />
-                            <field name="birthdate" />
+                            <field name="birthdate" required="1" />
                             <field name="gender" />
                             <field name="email" />
                             <field name="phone" />