Fixed style-lint errors for `panels.scss`

What does this MR do?

Fixes style-lint errors for app/assets/stylesheets/framework/panels.scss. The error has been fixed by removing @extend from SCSS all together in favor of setting corresponding base CSS class directly on HTML elements.

Affected templates obtained with

grep -rl --exclude-dir={node_modules,.git} "card-slim" .         
./app/views/projects/new.html.haml
./app/assets/javascripts/related_merge_requests/components/related_merge_requests.vue
./app/assets/stylesheets/framework/panels.scss
./tmp/cache/vue-loader/1323dd737571c80e5dde4ded651d8001.json
./.idea/workspace.xml

.xml and .json files were ignored.

Affected places:

Element	Before	After
new.html.haml
related_merge_requests.vue

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #59875 (closed)