Audit CSS utils being used as selectors in SCSS

In !154442 (merged), we initially added a script that checked whether CSS utils were being used as selectors in GitLab's SCSS. We did not keep the script, but here are its latest findings:

• [-] app/assets/stylesheets/components/whats_new.scss includes gl-spin: False positive (.gl-spinner-container).
• [-] app/assets/stylesheets/framework/animations.scss includes gl-spin: False positive (.gl-spinner).
• [-] app/assets/stylesheets/framework/buttons.scss includes gl-spin: False positive (.gl-spinner).
• app/assets/stylesheets/framework/common.scss includes gl-font-sm: !158830 (merged).
• app/assets/stylesheets/framework/common.scss includes gl-font-base: !158830 (merged).
• app/assets/stylesheets/framework/common.scss includes gl-font-lg: !158830 (merged)
• [-] app/assets/stylesheets/framework/common.scss includes gl-line-height-1: False positive as it's actually gl-line-height-14, but it should be moved to Tailwind anyways -> gitlab-ui!4384 (closed).
• [-] app/assets/stylesheets/framework/dropdowns.scss includes gl-spin: False positive (.gl-spinner)
• [-] app/assets/stylesheets/page_bundles/_pipeline_mixins.scss includes gl-spin: False positive (.gl-spinner).
• app/assets/stylesheets/page_bundles/trial_discover_page.scss includes gl-flex-basis-third: !158830 (merged).
• app/assets/stylesheets/page_bundles/trial_discover_page.scss includes gl-flex-basis-half: !158830 (merged).
• app/assets/stylesheets/pages/commits.scss includes gl-text-truncate: !158830 (merged).
• app/assets/stylesheets/utilities.scss includes gl-hover-border-gray-100: !158830 (merged).
• app/assets/stylesheets/utilities.scss includes gl-sm-mr-0: !158830 (merged).
• ee/app/assets/stylesheets/components/ai_agent_chat.scss includes gl-display-flex: !158830 (merged).

We need to audit those findings to get rid of potential CSS utils selectors in the stylesheets.