Add disabled option to checkboxes

[jrbarnes9]

The use of checkboxes in filters requires the addition of a disabled state for items which return zero result counts.

• $Grey35 can safely be used for the checkbox and label as WCAG2.1 states "disabled interface need not meet contrast minimum" (see incidental)
• aria-disabled could be applied to the label and input to supplement the standard <input ... disabled /> MDN reference
• cursor should be set to cursor: not-allowed; for label and input.

As seen in

and

@jondewijones following this item for use in search.

Originally posted by @armstrongb in https://github.com/ONSdigital/design-system/discussions/13#discussioncomment-2559997

[jrbarnes9]

@yatesn New request from DP

[jrbarnes9]

Resolved in #2187