Resolve "Inconsistent hover behavior on navbar items"

What does this MR do?

Fixes inconsistency regarding style when hovering over items in top navbar where all the items, except the Projects and Groups drop-downs, have smooth changing of background color.

Demo videos (click to play)

Before	After
26345-before	26345-after

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/59117

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• ~~Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.~~
• Security reports checked/validated by reviewer

Closes #59117 (closed)