Enable UnnecessaryMantissa in scss-lint

What does this MR do?

Enable UnnecessaryMantissa rule in scss-lint.

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

No.

Why was this MR needed?

To improve SCSS maintainability and consistency.

Screenshots (if relevant)

n/a

Does this MR meet the acceptance criteria?

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

What are the relevant issue numbers?

Part of #23485 (moved)