Update "Which browsers we support"
Update "Which browsers we support"

Add reference to GOV.UK Frontend browser support (since it was defined looking at analytics data on GOV.UK) and it reflects the current state of frontend applications that serve pages to the general public.

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs