Remove support for sx from the Header component.

Checklist

• Remove usage of sx from component
• Remove usage of SxProp from component, in particular from component props
• When necessary, remove BoxWithFallback usage that uses sx
• When necessary, remove Box usage usage that uses sx
• Remove any code that references sx concepts, in particular BetterSystemStyleObject
• If the component has tests that reference sx, remove them
• If the component has stories that reference sx, remove them
• If the component has sx usage downstream, add adapter to the @primer/styled-react package
• If you created an adapter for the component, update usage in github/github-ui to use the component from @primer/styled-react (if needed)