Update backwards compatibility guidance

[NoraGDS]

What

We will be removing backwards compatibility in the next breaking release - v13.0.0.

The guidance for backwards compatibility needs updating: https://govuk-prototype-kit.herokuapp.com/docs/backwards-compatibility

Why

The instructions say you will update to 7 or above, but following them will update you to the latest version. It's likely the instructions might not work.

Linked to removal of v6 support: https://github.com/alphagov/govuk-prototype-kit/issues/1308

Linked to release v13.0.0

Who needs to be involved

Content designer, interaction designer, developer

Done when

• [x] team agrees on removing compatibility
• [x] agreement on what changes to guidance required
• [x] guidance reviewed by content designer
• [x] guidance reviewed by team
• [ ] guidance updated as part of v13.0.0 release
• [ ] guidance unpublished from website

[NoraGDS]

Draft guidance for input: https://docs.google.com/document/d/1eBZO6V19WF43TlXgmGBvzlR2fymQc37rTCcRyG729BE/edit#

Team decided on Tuesday 28 June that we will no longer support backwards compatibility and users need to upgrade to v13. We will keep the guidance for internal support documentation.

[NoraGDS]

Content for backwards compatibility page: https://docs.google.com/document/d/1eBZO6V19WF43TlXgmGBvzlR2fymQc37rTCcRyG729BE/edit?pli=1# This content will be published along with v13.

The content currently published will become internal support documentation.

[NoraGDS]

this work is now covered in the epic