Drupal 9 to 10 update guide should mention the upstream

[kirrrk]

Issue

• The Drupal 9 to 10 update guide should clarify that it is only applicable for sites using the drupal-composer-managed upstream.

Suggested Resolution

• Add a step to the guide instructing the customer to check the sites upstream to verify that it is not using a deprecated upstream.
• Link to the guide for switching a site from a deprecated upstream to the current upstream: https://docs.pantheon.io/switch-drupal-recommended-upstream

Additional information

For customers to update from Drupal 9 to 10 successfully, they need to be on the current drupal-composer-managed upstream. This is not mentioned in our upgrade guide (https://docs.pantheon.io/drupal-10#update-a-drupal-9-site-to-drupal-10), leading many customers to encounter issues and reach out to support. Support also references this document, so it may not be clear to support staff that the customer has missed a step.

Notably, the update guide links to this on step 3: https://docs.pantheon.io/upgrade-drupal-with-ic-to-latest#set-drupal-core-version If you scroll up on this page to the "Will this guide work for you section?" (https://docs.pantheon.io/upgrade-drupal-with-ic-to-latest#will-this-guide-work-for-your-site), it does state that the site needs to be on the drupal-composer-managed upstream to follow the guide but it does not link to the guide for switching to the current upstream (https://docs.pantheon.io/switch-drupal-recommended-upstream).