chore: update project to be consistent with beram-drupal/docker-drupal-skeleton

- update drupal core to 8.8.5
- update PHP to 7.4
- use mglaman/phpstan-drupal instead of bare phpstan
5 jobs for chore/update-drupal-skeleton in 2 minutes and 57 seconds
Status Job ID Name Coverage
  Tests
passed nightly:outdated #516447919
docker

00:00:45

passed nightly:security_checker #516447918
docker

00:00:46

passed nightly:tests #516447920
docker

00:02:56

passed shellcheck #516447917
docker

00:00:45

passed tests #516447916
docker

00:01:02