Change top nav menu icon to 'hamburger'

What does this MR do?

Based on #332532 (closed), we're wanting to change the icon of the top nav dropdown from dot-grid to hamburger.

Screenshots (strongly suggested)

How to test?

You'll need to enable the :combined_menu feature flag. Then visit any page and notice the top nav menu icon has changed.

Before	After

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 changes are behind a 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.
• This change is backwards compatible across updates, or this does not apply.

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.)
• I have tested this MR in all supported browsers, or it's not needed.

Related to #332532 (closed)