Moved .toggle-mobile-nav to header.scss

What does this MR do?

Even though functionally, the mobile nav toggle relates to the left-side panel, it still resides in the breadcrumbs component. This MR moves the styling of it to header.scss that is responsible for styling the breadcrumbs. This is to avoid the styles clash in the course of the left-side panel refactoring

Screenshots (strongly suggested)

The look of the toggle hasn't been changed and the screenshots below have been added to confirm that.

Desktop	Mobile

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because the change is a developer-facing.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

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