Fixed `scss/at-extend-no-missing-placeholder` style-lint error for projects.scss

What does this MR do?

For #10951 (closed) this MR fixes

   9:3  ⚠  Expected a placeholder selector (e.g. %placeholder) to be used in @extend

style-lint error for ee/app/assets/stylesheets/pages/projects.scss. The error has been fixed by removing .protected-environments-list from SCSS all together and its appearances in templates replaced directly with .protected-branches-list that .protected-environments-list extended.

Before	After

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