Upgrade X-Gov 11ty plugin to version 6 and resolve breaking changes

[jeff-horton-ho-sas]

Code change

I can confirm:

Accessibility considerations

• [X] This change might impact accessibility and is not covered by automated aXe tests - manual testing has been performed

This upgrades X-Gov 11ty plugin to v6, which inlcudes changing to v5 of the gov.uk design system. Automated axe tests are passing, and a manual accessibility check on a sample of each layout used has been performed.

Upgrade guide: https://x-govuk.github.io/govuk-eleventy-plugin/upgrading/5-to-6/

• [X] Move styling config to a .scss file
• [X] Header configuration has changed
• [X] url fllter no longer needed