Suspend direct Drupal 8 support

[mglaman]

Drupal 8 is now EOL: https://www.drupal.org/psa-2021-11-30.

Earlier this year I added support for Drupal 9, which included a delicate dance to run our tests and build integration between Drupal 8 and Drupal 9. (See #157).

My proposal is that when beginning Drupal 10 support, drop Drupal 8. This makes my life easier. This may not even be necessary right now and may only be in the coming months.

Proposal:

• main branch is for 1.2+
• 1.1.x branch to support back port of fixes for D8 folks if ever needed
• Add conflict on first 1.2 release for drupal/core: < 9.0
• May not be necessary, but allows dropping Drupal 8 from CI.