Refactor PipelineSchedules services

What does this MR do and why?

This MR prepares a future improvement in PipelineSchedule validations by

• Creating Ci::PipelineSchedules::CreateService
• Adding error messages to the model in both Ci::PipelineSchedules::* services
• Using Ci::PipelineSchedules::UpdateService in a missing place

The changes are behind the FF ci_refactoring_pipeline_schedule_create_service (#416359 (closed)).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.