[FIX] hr_timesheet: timesheet rows in task have broken mobile layout

This commit https://github.com/odoo/odoo/commit/91fb15510ff549e7544f6858f7979b57fae875e7 hasn't been forward ported correclty.
Should use .col-6 instead of .col-xs-6 since Bootstrap 4.

Related to task 1929034

closes odoo/odoo#30787