Remove D8 for Drupal Composer upstream

[jms-pantheon]

Manually Migrate Sites to Pantheon - Remove old reference to Drupal 8 upstream in favor of Drupal Composer Managed

Release:

• [x] When ready

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Post Launch

Do not remove - To be completed by the docs team upon merge:

• [ ] Redirect /old-path/ => /new-path/ (if applicable)
• [ ] Include/exclude pages ^ respectively within docs search service provider (if applicable)
• [ ] For Heroes - add a props post to the discussion board.
• [ ] Remove from the project board

[pantheon-decoupled[bot]]

:zap: Deployed with Pantheon Decoupled

This build was successfully deployed with Pantheon. You can track the build logs here.

:eyes: Preview: https://pr-8728-documentation.appa.pantheon.site :hammer_and_wrench: Manage in Pantheon: https://dashboard.pantheon.io/site/2b30153f-e8b1-4427-b076-6109e704ba5d/overview

[stevector]

Assigning to myself as I now have enough info from a convo in Slack.