[IMP] core: rename upgrades-paths to upgrade-path

Since this option is soon to be used, this rename is a last tweek
to make it more logical to use since it will point to a single
upgrade dir most of the time.

closes odoo/odoo#44593

X-original-commit: 1c8e2809
Signed-off-by: Xavier Dollé (xdo) <xdo@odoo.com>