Update the "Here's how you know" banner to align with latest USWDS

[h-m-f-t]

Issue description

https://designsystem.digital.gov/components/banner/ has newer, better text describing how people can know they're on an official US government site. They have good .gov language! We should use it.

Acceptance criteria

• Update _includes/header.html with the content at https://designsystem.digital.gov/components/banner/

Additional context

Updating USWDS should also pull in these changes, which we might consider doing instead.

This issue is for get.gov. There is another issue for manage.get.gov, since they are separate sites.

Links to other issues

• Update the "Here's how you know" banner to align with latest USWDS manage.get.gov#2145

[vickyszuchin]

This may already be covered when we update USWDS