fix(component-header): keep gold underline on open dropdown, remove …

[davidornelas11]

…on close

Description

Gold underline on dropdown menu has different behavior than unity-bootstrap-theme, this pr matches that unity behavior by altering css to keep gold underline

Links

• Unity reference site
• UDS-1446
• Adobe mockup

Checklist

• [x] Unity project successfully builds from root yarn install & yarn build
• [x] Commits do not contain multiple scopes
• [x] Add/updated examples
• [x] Add/updated READMEs/docs
• [x] No new console errors
• [x] Accessibility checks

Browsers

• [x] Chrome
• [x] Safari
• [ ] Firefox
• [x] Edge