Fix SASS warnings due to deprecated functions

[acelaya]

Migrate from SASS' global darken function, which is deprecated, to the sass:color's module color.adjust function.

This solves two warnings reported by SASS:

DEPRECATION WARNING: Global built-in functions are deprecated and will be removed in Dart Sass 3.0.0.
Use color.adjust instead.

More info and automated migrator: https://sass-lang.com/d/import

   ╷
28 │ $link-hover-color: darken($link-color, 15%);
   │                    ^^^^^^^^^^^^^^^^^^^^^^^^
   ╵
    lms/static/styles/moodle_pages/moodle_pages.scss 28:20  root stylesheet

DEPRECATION WARNING: darken() is deprecated. Suggestions:

color.scale($color, $lightness: -30%)
color.adjust($color, $lightness: -15%)

More info: https://sass-lang.com/d/color-functions

   ╷
28 │ $link-hover-color: darken($link-color, 15%);
   │                    ^^^^^^^^^^^^^^^^^^^^^^^^
   ╵
    lms/static/styles/moodle_pages/moodle_pages.scss 28:20  root stylesheet

The migration from darken to color.adjust is documented here https://sass-lang.com/documentation/modules/color/#darken. I used that to decide what parameters to pass to color.adjust.