[FIX] gamification: use context_today instead of today

`fields.Date.today` doesn't give date based on TZ which might be wrong
in some cases as it doesn't respect TZ.

With this commit, we are using `fields.Date.context_today` as default date.

closes odoo/odoo#75600

Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>