[FIX] core: merge scripts found in `--upgrade-path`

Don't limit on the first directory found in the upgrade-path.

It follow the same behavior regarding scripts found in the `maintenance`
directory and the ones in the local `upgrades` directory of modules.

opw-2868713

closes odoo/odoo#104887

Signed-off-by: Christophe Simonis <chs@odoo.com>