Replace deprecated sass color operation (notification badges in nav)

What does this MR do and why?

Replaces deprecated sass color operation only affecting on notification badges in the nav (on help and on user avatar).

Closes #365439 (closed)

Follows up and fixes up !27626 (merged)

Screenshots or screen recordings

before	after

You can see no regression at https://youtu.be/BTQLzVCMQn4:

How to set up and validate locally

1. Comment out or remove line 1: https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/views/layouts/header/_user_notification_dot.html.haml#L1
2. Just visit /.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.