Migrate GitHub actions to GitLab CI

Note that if the phpunit job depends on this, you can turn it off temporary in GitLab CI setting SKIP_PHPUNIT to 1, so it doesn't run. I see it's currently being run and failing https://git.drupalcode.org/project/search_api_solr/-/jobs/450099

The main issue is that the tests require different Solr versions. Using Github actions we could simply use docker to start them.
That seems way more complicated using the GitLab CI and the drupal templates.

Note that we are not limiting the use of external images for solr and the usage of docker.

The only caveat is that we do not allow docker-in-docker for security reasons, but you can use the alternatives listed here: https://docs.gitlab.com/ee/ci/docker/using_docker_build.html#docker-alte...

Here is an example of a project using "buildah": https://git.drupalcode.org/project/drupalci_environments/-/blob/dev/.git... and calling "docker" commands inside the scripts.