Refactor confidential and hidden issue icons on boards to use CSS utility classes

Summary

Follow-up to !69323 (comment 666901694)

app/assets/stylesheets/page_bundles/boards.scss#L251 can be refactored to GitLab UI utility classes

Improvements

Reduce CSS bundle size

Risks

Low risk

Involved components

• app/assets/stylesheets/page_bundles/boards.scss
• app/assets/javascripts/vue_shared/components/issue/related_issuable_item.vue#L90
• spec/frontend/boards/board_card_inner_spec.js#L131
• spec/frontend/vue_shared/components/issue/related_issuable_item_spec.js