Remove support for Ruby Sass and LibSass

[querkmachine]

Closes #6242 and #6243. Part of #2637.

Warning

We're treating the removal of support for Ruby Sass and LibSass as a breaking change. This PR should not be merged until we're preparing to release Frontend v6.0.0.

Prior to merging, the branch protection rules for main will need to be updated to no longer require the Ruby Sass and LibSass tests to have been ran.

Changes

• Removes CI tests for Ruby Sass and LibSass (including node-sass).
• Removes deprecation warning shown to LibSass users.

[github-actions]

Other changes to npm package

diff --git a/packages/govuk-frontend/dist/govuk/settings/_index.scss b/packages/govuk-frontend/dist/govuk/settings/_index.scss
index 9cbf2779d..e46fb4637 100644
--- a/packages/govuk-frontend/dist/govuk/settings/_index.scss
+++ b/packages/govuk-frontend/dist/govuk/settings/_index.scss
@@ -20,17 +20,4 @@
 
 @import "links";
 
-// LibSass evaluates this as a single unquoted string, equivalent to
-// "true == false", whereas Dart Sass evaluates it as "true" == "false".
-// Because a string in Sass is truthy, LibSass will output the warning; Dart
-// Sass will not.
-@if #{true} == #{false} {
-  @include _warning(
-    "libsass",
-    "It looks like you may be using LibSass to compile your Sass. LibSass is " +
-      "deprecated and will not be supported by the next major version of " +
-      "GOV.UK Frontend. See https://sass-lang.com/libsass/ for more information."
-  );
-}
-
 /*# sourceMappingURL=_index.scss.map */

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Action run for 76a08b7

[github-actions]

📋 Stats

No changes to any distributed file sizes!

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Action run for 76a08b7

[romaricpascal]

Looks good to merge.

On top of main we'll need to change the protection rules of our other long-lived branches (feature/* and support/* branches).