Turn on the new type scale by default

[owenatgov]

What

Turn the new type scale on and remove the $govuk-new-typography-scale feature flag and set $govuk-typography-scale to be the new scale by default. Related to alphagov/govuk-design-system#2289

Why

So that the new type scale is available by default, like we said we'd do.

Who needs to work on this

Developers

Who needs to review this

Developers, tech writers

Done when

• We've removed all references to $govuk-new-typography-scale
• We've set the 'modern' type scale to be the default type scale
• We've removed the 'legacy' type scale

[owenatgov]

Can't really be done in ernest until #2797 is finished.