[FIX] hr_contract: Move scss in correct folder

File `calendar_mismatch.scss` was wrongly in a `css` folder
while it should be in a `scss` folder according to the guidelines.

And set the correct `type="text/scss"` instead of `type="text/css"`
for a .scss file.

closes odoo/odoo#34478

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