Drop mentions of being able to drop the `index` when using LibSass or Ruby Sass

[romaricpascal]

What

Remove the "LibSass 3.6.0 or higher, or Ruby Sass 3.6.0 or higher" part of the statement that follows Sass examples which import index files:

You do not need /index at the end of your import if you’re using Dart Sass, LibSass 3.6.0 or higher, or Ruby Sass 3.6.0 or higher.

Why

Highlighting benefits of using newer versions of deprecated libraries may encourage people to keep using LibSass and Ruby Sass.
We've deprecated our support for them and the Sass ecosystem has moved on to Dart Sass, so we'd want to encourage team to move in that direction.

Who needs to know about this

Developers, Content designers

Done when

• Statement of benefits for LibSass and Ruby Sass has been removed