[FIX] point_of_sale: add `forcecreate="0"` `pos_config_main`

Without this change, upgrades would generate the `pos_config_main`
record even in dbs with otherwise-well-organized structure.

Also, it might cause issues[^1] hard to solve otherwise.

[^1]: https://upgrade.odoo.com/web#active_id=421039&cids=1&id=421039&menu_id=107&model=upgrade.request&view_type=form



closes odoo/odoo#107716

Signed-off-by: Trinh Jacky (trj) <trj@odoo.com>