Prevent gl-new-dropdown rule override

What does this MR do?

This changes the CSS rules to prevent the .gl-new-dropdown class from the GlNewDropdown and the subsequent menu items from having their display rules changed from flex to block

Issue for the component migration: gitlab-ui#673 (closed)

Screenshots

I tested this with the only feature that uses the GlNewDropdown component, as well as some other features that still use the older one

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Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
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