Enable NestingDepth (level 6) on scss-lint

What does this MR do?

Enable NestingDepth (with max_depth of 6) on scss-lint.

Are there points in the code the reviewer needs to double check?

Changes in app/assets/stylesheets/pages/notes.scss would be checked.

Relevant codes are following:

• app/assets/stylesheets/framework/gitlab-theme.scss: 9b177bb7 !14190 (merged) (original: e694777c !13619 (merged)) by @annabeldunstone
• app/assets/stylesheets/framework/header.scss: 7312b870 !14573 (merged) by @annabeldunstone
• app/assets/stylesheets/pages/notes.scss: 58f78d05 !6755 (merged) by @nmrony

Why was this MR needed?

Deeper nesting is nothing good for developers.

Screenshots (if relevant)

n/a

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #39582 (closed)

Related to #23485 (moved)