Refactor govuk-frontend Sass imports

govuk-frontend 3.7.0 changes the way that Sass dependencies work, so
that CSS compiles faster [[1]].

This commit changes the Sass file for the tech docs gem to take
advantage of the new system, by importing the component `index`
stylesheets which don't include dependencies multiple times.

[1]: https://github.com/alphagov/govuk-frontend/blob/main/CHANGELOG.md#sass-now-compiles-faster

lib/assets/stylesheets/_govuk_tech_docs.scss

@@ -2,20 +2,18 @@ $govuk-assets-path: "/assets/govuk/assets/";
 
 // Include only the bits of GOV.UK Frontend we need
 $govuk-new-link-styles: true;
-@import "govuk/settings/all";
-@import "govuk/tools/all";
-@import "govuk/helpers/all";
+@import "govuk/base";
 
 @import "govuk/core/all";
 @import "govuk/objects/all";
 
-@import "govuk/components/footer/footer";
-@import "govuk/components/header/header";
-@import "govuk/components/inset-text/inset-text";
-@import "govuk/components/input/input";
-@import "govuk/components/tag/tag";
-@import "govuk/components/skip-link/skip-link";
-@import "govuk/components/warning-text/warning-text";
+@import "govuk/components/footer/index";
+@import "govuk/components/header/index";
+@import "govuk/components/inset-text/index";
+@import "govuk/components/input/index";
+@import "govuk/components/tag/index";
+@import "govuk/components/skip-link/index";
+@import "govuk/components/warning-text/index";
 
 @import "govuk/utilities/all";
 @import "govuk/overrides/all";