Use utility classes instead of CSS variables for deprecated design tokens in GitLab UI

GitLab UI still contains instances of CSS variables like var(--gl-text-color-subtle) that correspond to Tailwind utility classes such as @apply gl-text-subtle.

These variable usages were likely introduced during the design token migration and have not been updated since GitLab UI began using utility classes. This results in inconsistencies and makes the UI look visually outdated compared to GitLab.