Update for removal of Ruby Sass and LibSass support

[querkmachine]

Closes #558.

[!WARNING] These changes may need to be rebased or merged elsewhere depending on how we end up separating documentation for v5 and v6. It should probably not be merged just yet.

The process for determining Sass support going forwards has been added to the govuk-frontend repo's docs folder, as it feels more like developer process than user-facing documentation. https://github.com/alphagov/govuk-frontend/pull/6381

Changes

All of these changes are only in the current (not v4) version of the documentation:

• Update minimum installation version to 1.79.0, as per this comment.
• Update messaging to say we don't support any versions of Ruby Sass or LibSass, but maintain the migration documentation link.
• Remove conditional statements relating to Dart Sass versions older than this.
• Remove notice about needing to include index in import paths, as we no longer support Ruby Sass and LibSass (which required these).
• Remove index from import path example.
• Remove reference to LibSass in context of deprecation warnings.

This change is in the v4 version of the documentation:

• Update to clarify that v4 and v5 of GOV.UK Frontend support LibSass and Ruby Sass, but v6 does not.

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