Goodbye paper tax discs (DO NOT MERGE UNTIL OCT 1ST)

As of October 1st, paper tax discs will be abolished. This pull request updates three start pages to reflect this change to instead refer to 'vehicle tax' rather than 'tax discs'.

The affected start pages are:
https://www.gov.uk/tax-disc
https://www.gov.uk/make-a-sorn
https://www.gov.uk/check-vehicle-tax