Swap OncallRotations::BaseService to use ::BaseProjectService

What does this MR do and why?

Service classes should inherit from one of BaseProjectService/BaseGroupService/BaseContainerService. This MR migrates ee/app/services/incident_management/oncall_rotations/base_service.rb to inherit from BaseProjectService (and all impacted services).

This MR has no functional or user-facing changes.

Related issue: #348958 (closed) (sorta; I'm doing all grouprespond-owned services, but this issue is a follow-up regarding a particular service)

MR acceptance checklist

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