Drupal core 9.x.x releases should not attach API compatibility term

The initial work here is done. The API compatibility field will be blank for releases associated with branches like 9.0.x, and tags like 9.0.0. 8* will continue as it did before. One notice on viewing the release page has been fixed.

I’m testing basic (legacy) packaging to exercise this a bit more before calling this done. Since there are assumptions about this field having a value everywhere, there will be followups.

Adding todo list

#3087220: Migrate project_release_supported_versions to store “branch” instead of API/Major is now deployed, and I did some spring cleaning on the core-specific Views used on https://www.drupal.org/project/drupal, which now has “Drupal core 9.0.x-dev” listed.

A bit more work is needed on the core page’s release Views, running through the scenarios as D9 progresses. For example, '<h3>Drupal 8.[field_release_version_minor].0 is coming soon. Help us test it today!</h3>' won’t work forever.

A bit more work is needed on the core page’s release Views, running through the scenarios as D9 progresses

This is ready for 9.0.0-alpha1, if not all of the releases.

Everything in this issue is now done.

Notable followups are #3089291: Group issue version “series” by branch instead of API compatibility & #3088642: Group usage statistics by “branch” instead of API compatibility