Fix missing data and class in top_nav_menu_item

• Requirements (issue)
• Tasks (epic)
• Full Context Spike MR (permanent draft)
• All MR parts ...
  • Fix missing data and class in top_nav_menu_item

What does this MR do?

This MR updates the top_nav_menu_item to actually render the css_class and data it gets from the view model. This fixes a number of things:

• Fixes "Leave Admin Mode" not working since it needs data-method="post"
• Fixes missing QA selectors

Screenshots (strongly suggested)

How to test?

• Make sure combined_menu feature flag is enabled
• Sign in to the admin user
• Visit the admin settings page and activate the Admin mode checkbox

• Hit Menu > Enter Admin Mode
• Type in password
• Hit Menu > Leave Admin Mode

Before (on master)	After (this MR)

Does this MR meet the acceptance criteria?

Conformity

• [ ] I have included a changelog entry, or it's not needed. (Does this MR need a changelog?) Not needed since behind feature flag.
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)

References

Related to #301143 (closed)