Deprecate the legacy organisation list and colour palette

[querkmachine]

What

Properly deprecate the following, in preparation for removing them in a future major release of GOV.UK Frontend:

• the legacy organisation list and colour palette
• the $govuk-new-organisation-colours feature flag

The $websafe parameter of the govuk-organisation-colour function has already been marked as deprecated.

Why

The legacy organisation list and colour palette has been replaced by a newer version. The legacy list will not receive updates and should be removed.

Who needs to work on this

• Developers

Who needs to review this

• Developers

Done when

• [ ] The legacy colour palette has been marked as deprecated
  • [ ] Should govuk-organisation-colour return a warning if the legacy palette is being used?
• [ ] The $govuk-new-organisation-colours feature flag has been marked as deprecated