Update to Drupal 8.9.20. For more information, see https://www.drupal.org/project/drupal/releases/8.9.20

[pantheon-upstream]

Update from Drupal 8.9.19 to Drupal 8.9.20.

Before merging this PR, check the build results on CircleCI, and then visit the test site and confirm that the correct version of Drupal was, in fact, installed and tested.

Optionally, you may also create your own test site:

• Create a new Drupal 8 site on Pantheon.
• When site creation is finished, visit dashboard.
• Switch to "git" mode.
• Clone your site locally.
• Apply the files from this PR on top of your local checkout.
  • git remote add drops-8 git@github.com:pantheon-systems/drops-8.git
  • git fetch drops-8
  • git merge drops-8/update-8.9.20
• Push your files back up to Pantheon.
• Switch back to sftp mode.
• Visit your site and step through the installation process.