Deprecate smaller font sizes on mobile in the font scale

[owenatgov]

What

Deprecate the following as options for our font mixins (govuk-font and govuk-typography-responsive) from the typography scale:

• 14 (14px on large screens, 12px on small screens) as an option at all
• 16 dropping down to 14px on small screens
• 19 dropping down to 16px on small screens

Why

To communicate to service teams that we will be removes these as options in the next breaking release (5.0).

Part of alphagov/govuk-design-system#2289

Who needs to work on this

@owenatgov

Who needs to review this

Developers

[owenatgov]

Closing this PR as we're reviewing when and how we deprecate these parts of the font scale as part of a wider release plan for the font scale work.