This brings the gem up to govuk-frontend 6.4.0, including the GOV.UK rebrand assets, and clears up the drift that had built up between the gem, its lockfile and the vendored files.
This is a breaking change. Before taking it, apps need to move from sassc-rails to dartsass-sprockets, update govuk_design_system_formbuilder to a version matching govuk-frontend 6, and replace the markup they put in the header_content slot. The README covers all three.
It also fixes govuk-frontend's JavaScript, which has never run in any of the apps because nothing was setting the govuk-frontend-supported class. Error summaries will now take focus when a form fails validation.