[FIX] pos_discount: correctly get discount amount

Activate a language which has comma as decimal separator
Go into POS settings, activate 'Global Discount'.
Open POS session, add some product and use global discount.
Input some number with decimals and confirm.
Discount will be not applied

This occur because of javascript type coercion:
 Math.min(100, "11.0") -> 11
 Math.min(100, "11,0") -> NaN

So in locales with thousands separator the statement would produce
different results. Forcing float parsing fix the issue

opw-2243482

closes odoo/odoo#50558

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>