Upgrade to use govuk-frontend v5

[kr8n3r]

What’s changed

• updated asset pipeline and packages used to be able to import govuk-frontend v5
• new govuk_frontend_all entry point that imports all.bundle from govuk-frontend
• updated javascript and sass compilers
• updated location paths for govuk-frontend
• updated markup as per govuk-frontend release notes

Identifying a user need

These changes serve two purposes:

1. bring tech-docs frontend dependencies up to date from a maintenance / upgradability point of view
2. bring with it some WCAG 2.2 compliance govuk-frontend offers so that when it comes to assessment / audit, we're in a better position

How to review / test

See individual commits for more detail Test in a downstream project:

• clone tech-docs-gem and switch to this branch
• create a govuk_frontend.js file in project’s source/assets/javascripts directory
• add //= require govuk_frontend_all into it
• point to local gem https://github.com/alphagov/tech-docs-gem/tree/use-govuk-frontend-v5?tab=readme-ov-file#use-your-own-tech-docs-template-website
• run bundle install
• run bundle exec middleman server or build