[IMP] base: Change default signature for admin demo data

PURPOSE

Currently, the signature of the admin defaults to 'administrator'. Odoo
does not take into account the actual name of the user.  The onboarding
experience would be better if the signature was personalized
for the user.

SPECIFICATIONS

The default signature of the admin is his name (Mitchell Admin) instead of 'Administrator'.

Task-2416715

closes odoo/odoo#63914

Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>