API: Rename services to integrations
### Problem to solve
In https://gitlab.com/gitlab-org/gitlab/issues/39129 we decided that we want to rename `project services` to `integrations`. This also includes to rename the [services endpoint](https://docs.gitlab.com/ee/api/services.html).
#### There are a few questions we need to answer before we can come up with a solution:
- [How can we rename the endpoint without introducing breaking changes?](https://gitlab.com/gitlab-org/gitlab/issues/198358#note_275258195)
- [How long to we need to wait before we can deprecate the services endpoint?](https://gitlab.com/gitlab-org/gitlab/issues/198358#note_275248183)
- How can we document the changes without causing confusion
<!-- triage-serverless v3 PLEASE DO NOT REMOVE THIS SECTION -->
*This page may contain information related to upcoming products, features and functionality.
It is important to note that the information presented is for informational purposes only, so please do not rely on the information for purchasing or planning purposes.
Just like with all projects, the items mentioned on the page are subject to change or delay, and the development, release, and timing of any products, features, or functionality remain at the sole discretion of GitLab Inc.*
<!-- triage-serverless v3 PLEASE DO NOT REMOVE THIS SECTION -->
epic