Update GOV.UK Prototype Kit release note to include GOV.UK Frontend 3.0.0 changes

[NickColley]

Once GOV.UK Frontend v3.0 has been released, we need to add CHANGELOG instructions for people trying to update from Prototype Kit version v8 and below.

We should be able to simplify the main release notes as some of it may not apply.

Why

So that prototype kit users are not stuck on older versions.

Dependencies

• Release GOV.UK Frontend v3.0 🚀 Release GOV.UK Frontend v3.0 🚀 govuk-frontend#1485

Done when…

• Changelog has been updated with relevant GOV.UK Frontend 3.0.0 update instructions