[meta] Review scss-lint rules

Description

We should review and revise the sass lint rules

This would help us lighten the load for MR miniboss and endbosses which would help them better focus on getting MRs merged!

Rules to add

• All color values should be stored in variables (gitlab-foss#23500 (closed))
• Enforce nesting (Set as a warning as we have a lot of these deep nests currently but should start moving away from it) (gitlab-foss#23501 (moved))
• TrailingWhitespace gitlab-foss!7077 (merged)
• SpaceAroundOperator gitlab-foss!7078 (merged)
• SpaceAfterVariableColon gitlab-foss!7080 (merged)
• SingleLinePerSelector gitlab-foss!7081 (merged)
• SelectorFormat
• PseudoElement gitlab-foss!7083 (merged)
• NameFormat
• IdSelector
• NestingDepth (level 4)