Update panel guidance to advise not using long reference numbers for new typography scale

[owenatgov]

What

Update the guidance for the panel component to suggest a limit on reference number length.

Should not be completed until alphagov/govuk-frontend#2421 is merged and published from govuk-frontend and the version containing this change is applied to govuk-design-system.

Why

During testing of the govuk-frontend components against the new typography scale, we noticed that the panel component now looked quite squashed on small screens with the text no longer dropping down to a smaller size on small screens. There is a separate issue to reduce the padding of the panel component to mitigate this, but we think we can mitigate this even further with improvements ot the guidance.

Who needs to work on this

Content designers

Who needs to review this

Content designers