Hook up update schedule mutation

What does this MR do and why?

Part of the refactor of pipeline schedules to Vue/GraphQL. Hooks up the update schedule mutation and adds ability to update and destroy variables.

Screenshots or screen recordings

How to set up and validate locally

1. Enable the feature flag Feature.enable(:pipeline_schedules_vue)
2. Visit Project > Build > Pipeline schedules
3. Try creating a pipeline schedule with different values and test out creation
4. Then edit the pipeline schedule and ensure the values are updated correctly (try removing variables etc.)

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.

Related to #385101 (closed)