use new $govuk-new-organisation-colours map

`legacy-organisation-colours` map is getting
deprecated, so a new one needs to be used

[Deprecation](https://github.com/alphagov/govuk-frontend/releases#:~:text=Migrate%20to%20the%20new%20organisation%20colour%20palette)

[New palette released in v5.5.0](https://github.com/alphagov/govuk-frontend/releases#:~:text=in%20our%20documentation.-,New%20features,-We%27ve%20updated%20the)

Author: kr8n3r

lib/assets/stylesheets/_govuk_tech_docs.scss

@@ -7,6 +7,9 @@ $govuk-suppressed-warnings: (
 );
 $govuk-assets-path: "/assets/govuk/assets/" !default;
 
+// legacy palette is getting deprecated so new one needs to be used
+$govuk-new-organisation-colours: true;
+
 // Include only the bits of GOV.UK Frontend we need
 $govuk-new-link-styles: true;
 @import "govuk/base";