[FEATURE] Upgrade to X-Gov 11ty plugin v6 / Gov.uk design system v5

[jeff-horton-ho-sas]

What is the feature or enhancement you are suggesting? Upgrade to the latest versions of X-Gov 11ty plugin and design system components.

What would be the benefit of doing this? Keep key dependencies up to date, uses improved version of the design system.

Additional information There are some breaking changes to the plugin configuration, and design system changes need to be reviewed for accessibility.

Please confirm the below

• [X] I have checked through the issues on the repository and this has not already been suggested