Let users know about removal of v6 support

[lfdebrux]

What

Create and share comms about removing v6 support in the next major release of the GOV.UK Prototype Kit.

The comms should include information about what's going to happen, that we're not going to keep helping users with issues related to v6, and what to do about it.

Note: we might want to be able to give a timeline for removal, in which case we'll need to decide on one!

Part of #1295.

Why

We want to make sure users are aware and prepared for us to drop support for frontend toolkit/elements.

Who needs to work on this

Content designer

Who needs to review this

PM

Done when

• Comms drafted
• Team reviews and agrees comms
• Talked to Imran about sharing comms beyond xgov Slack (Ruth)
• Comms shared
• Warning added to update script

[NoraGDS]

Draft comms happening here: https://docs.google.com/document/d/17qFc-crDmgT2G7LSAgtG1jc9DLPdsCjy2qL9jvJqaUk/edit#

[NoraGDS]

A couple of outstanding decisions to make so this can be ready and we can also update the guidance here: https://govuk-prototype-kit.herokuapp.com/docs/backwards-compatibility