Fix checkmark position on dropdowns

What does this MR do?

Possible solution for #215960 (closed)

removing :not(.gl-dropdown), and de-nesting .dropdown-item from .dropdown-menu. Means gl-new-dropdown's styles are more specific than the default gitlab dropdown-item.

Could potentially change specificity in case of existing overrides. For any .dropdown-items that have another class applied that class could now take precedence over the dropdown-menu styles. However every instance I can find is adding utility classes, which are already intentionally more specific than nested .dropdown-item (mostly utils using !important)

AND most regular dropdown the styles are duplicated in the .dropdown-menu li button which includes the exact same mixin

Screenshots

Before	After

And gl-new-dropdown still looks ok! (height issue is unrelated)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done