Deprecate remaining `all.scss` in favour of `index.scss`

[romaricpascal]

What

Add a deprecation warning in govuk/all.scss file to let people know that we'd prefer them to use govuk/index.scss instead, similarly to what's been done for the all.scss for the different layers of our CSS.

This should be the last all.scss file without deprecation remaining, but worth having a look that:

• all all.scss files have a corresponding index.scss file
• all @import in govuk-frontend use the index.scss file and not all.scss (still explicitely using a trailing /index to keep compatibility with older versions of Sass we still support)

Update the exports and sass entries in our package.json so that projects that the update is transparent for people using the npm package.

Update the Frontend docs for any reference to the govuk/all.scss file, replacing it with govuk/index.scss

Why

This is a step in our migration to using index.scss files, with the benefits listed in the first PR for that change.

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

• [ ] A deprecation message is added to the govuk/all.scss
• [ ] The package.json file has been updated
• [ ] The Frontend docs have been updated