Update the GOV.UK Prototype Kit to use GOV.UK Frontend v4.0.0

[domoscargin]

This uses the govuk-frontend v4 pre-release.

I've gone through the CHANGELOG for v4: https://github.com/alphagov/govuk-frontend/blob/main/CHANGELOG.md and there were only a couple of minor things that needed updating.

We can merge this when we have replaced the pre-release with the actual release.

Closes #1063

Summary of findings

• The kit doesn't have any accordions. The step by step navigation uses its own styles and scripts to function.
• No use of the govuk-main-wrapper mixins
• No use of the $govuk-border-width-form-element-error variable
• The kit does have summary lists, but none of them have an empty actions span
• The kit doesn't use navigationClasses on the header
• The kit doesn't include conditionally revealed content (other than on the step by step, which handles that itself)
• There are no character counts
• The kit doesn't import individual JavaScript modules
• The kit has no error message spans
• The kit had some hint spans which have been updated
• No uses of the Sass iff function
• No use of govuk-tag--inactive
• No cookie banner
• The kit doesn't import individual Sass files from the core or override layers
• Footer displays correctly
• The kit had some SVGs with role="presentation" which have now been updated

[lfdebrux]

Thanks for being explicit and listing out the things you checked!

I think this was added to the release since you looked at it, but I checked and we're using the Nunjucks macro for the skip link, so we don't have to do anything to include JavaScript for the skip link.

[lfdebrux]

I also checked that we don't use HTML date inputs anywhere, so we don't need to worry about changing date input from type="number" to inputmode="numeric".

[36degrees]

A few docs related suggestions, but otherwise the changes look sensible to me.