Split `.gitlab-ci.yml` into multiple includes

What does this MR do?

This tries to break the complexity of .gitlab-ci.yml by splitting it into multiple includes that provide functional parts for different test scenarios and are "almost independent".

EE version: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10854

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Style guides
• Database guides
• Separation of EE specific content

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/58521.