Drupal Composer moved Pantheon upstreams

[jms-pantheon]

Summary

The old pantheon-systems location of drupal-composer-managed is now deprecated.

Release:

• [x] When ready

---

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-8746-documentation.appa.pantheon.site :hammer_and_wrench: Manage in Pantheon: https://dashboard.pantheon.io/site/2b30153f-e8b1-4427-b076-6109e704ba5d/overview

[stevector]

This PR inspired https://github.com/pantheon-systems/documentation/issues/8912

[stevector]

Same question here https://github.com/pantheon-systems/documentation/pull/8728/files#r1543120964

[stevector]

I added the blocked label because I'd like to get this question answered: https://github.com/pantheon-systems/documentation/pull/8728/files#r1543120964