fix: fix Dropdown Menu top level item properties #11412

[ncoden]

Description

This pull request fix misunderstandings in the effects Dropdown Menu "top-level" items settings have and remove an unexpected visual change introduced in #11377.

Changes:

• Only apply top-level styles to top-level items. This increase specificity but this markup structure is alreaddy assumed elsewhere in the component.
• Remove the default $white background on top-level items. It was never applied before Define dropdown menu background in foundation-dropdown-menu mixin #11377 and could now cause visual changes.

• Introduced in Define dropdown menu background in foundation-dropdown-menu mixin #11377
• Closes Default CSS for Foundation 6.5.2-rc.2 breaks menu background (regression from 6.4.3) #11412
• [Compatible with v6.5.0]

Types of changes

• Documentation
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing
  functionality to change)

Checklist (all required):

• I have read and follow the CONTRIBUTING document.
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• The pull request title is descriptive.
• The template is fully and correctly filled.
• The pull request targets the right branch (develop or develop-v...).
• My commits are correctly titled and contain all relevant information.
• My code follows the code style of this project.
• I have updated the documentation accordingly to my changes (if relevant).
• I have added tests to cover my changes (if relevant).
• All new and existing tests passed.