Updates for GOV.UK Frontend v5.0.0

[colinrotherham]

This PR contains all of the updates for the v5.0 release. It is the culmination of the following PRs which have been reviewed individually:

• Preview v5: Update template with "supported" check #317
• Preview v5: Update package paths for govuk-frontend@5 #318
• Preview v5: Archive the announcement page #325
• Preview v5: Update examples using ES modules #327
• Preview v5: Content changes #332
• Preview v5: Remove references to IE8 in code #334
• Preview v5: GitHub Actions fixes #335
• Preview v5: Update application paths for govuk-frontend@5 #336
• Preview v5: Add explanation that individual initialisation will throw errors #352
• Add new page describing our browser support strategy #356
• Add changes to v5 doc #357
• JavaScript documentation pre-release updates #358
• Mention availability of a built CSS file in the package #359
• Prevent confusion from preview site linking to release site #360
• Host v4 in its own folder #384
• Move 'Changes to GOV.UK Frontend v5.0.0' to top of menu #387
• Include file manifest.json when copying and testing assets #394
• Few typos and tweaks for v5 #396
• Placeholder for v5 content changes #399
• Add drafted content to the repository #403
• Improve ES module and alternative format documentation #404

Version numbers

To support version numbers in package-lock.json from GitHub branch previews, initial commits include:

1. Upgrade to Node.js 18
2. Convert to "lockfileVersion": 3 for GitHub branch versions
3. Install GOV.UK Frontend v5.0.0 preview

Upgrading package-lock.json to "lockfileVersion": 3 includes dependencies packages field by default:

"node_modules/govuk-frontend": {
  "version": "5.0.0-testing",
  "resolved": "git+ssh://git@github.com/alphagov/govuk-frontend.git#0f5c7975f759692ae91f88cc2227a9cefb307f17",
  "dev": true,
  "license": "MIT"
},

So we can continue accessing version in our SassDoc helpers:

--- A/lib/sassdocs_helpers.rb
+++ B/lib/sassdocs_helpers.rb
@@ -132,7 +132,7 @@
     # Get the current version of GOV.UK Frontend from the package.
     package_lock_file = File.read("./package-lock.json")
     package_lock = JSON.parse(package_lock_file)
+    @govuk_frontend_version = package_lock["packages"]["node_modules/govuk-frontend"]["version"]
-    @govuk_frontend_version = package_lock["dependencies"]["govuk-frontend"]["version"]
     @govuk_frontend_version
   end
 end

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[colinrotherham]

@romaricpascal Updated the description with your PR and had to rebase

This branch uses Node.js 18 + "lockfileVersion": 3 so it conflicts every Dependabot update 😣

I'll split those changes out into another PR so we can prevent this for the next Monday:

• Use Node.js v18 + "lockfileVersion": 3 etc #353