Change name and value of dropdown-fa-input-color variable

We currently have $dropdown-input-fa-color: #c7c7c7; defined in https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/stylesheets/framework/variables.scss#L549

This is mainly used to display search and clear icons in some of our dropdown inputs, see:

The variable name should be changed since it indicates that this is related to Fontawesome which we are about to remove from our codebase. Also, the color value should probably changed to be aligned with our design system.