Rename service to integrations in API

What does this MR do?

Related to #333580 (closed)

It renames methods, variables and classes from service to integration in the API. It does not change any functional behavior in order to not introduce breaking changes.

I decided to not rename the API::Services class because the routes will still be /projects/:id/services. The definition can easier be found if the class name matches the REST resource name.

Does this MR meet the acceptance criteria?

Conformity

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