Add new letter rates for March 1, 2022.
Add new letter rates for March 1, 2022.

- second class postage will go up by 2 pence, plus VAT
- international postage will go up by 7 pence, plus VAT
- first class postage will go down by 6 pence, plus VAT

alphagov/notifications-api,alphagov/notifications-api