Rename project and references from drupal9ci to drupalci

[juampynr]

From https://github.com/Lullabot/drupal9ci/issues/87

Given that this project supports multiple Drupal versions, let's make the name version agnostic.

[deviantintegral]

I think the d.o infra team also refers to their testing infrastructure as drupalci.

Naming is hard.