REST API: Rename services to integrations (without making any functional changes)

Description

This is part of the effort to rename services to integrations &2504 (closed).

We need to rename all classes (and methods, variables if applicable) involved in the REST services API from "service(s)" to "integration(s)".

Note, that we should not introduce any functional changes to the API itself; so the endpoint will still need to be services, with all relevant field and argument names also preserved. This is because in API v5 we'd make the full breaking change of renaming the endpoint as outlined in the REST API docs.