Rename project_services to integrations in codebase

As part of the Mass-integration project, we decided in &2504 (closed) to rename project services to integrations.

To reflect the new name and in accordance with the Gitlab/NamespacedClass cop !51236 (merged):

• The project_services directory should be renamed to integrations
• The models should be namespaced with Integrations::, i.e. Integrations::Jira.

Checklist

• Rename classes namespaced Integrations::
• Remove Rubocop TODOs https://gitlab.com/gitlab-org/gitlab/-/blob/9a65f0bd374515fc8e720fbb6e4889653dfae527/.rubocop_manual_todo.yml#L1482-1531
• We should not break the usage metrics, details #201855 (comment 562526950)
• Manage Single table inheritance #201855 (comment 564903142)
• GraphQL considerations #201855 (comment 566482827)