Migrate timezone dropdown on pipeline schedules

What does this MR do and why?

Migrate outdated timezone selector to Vue Compo... (&8530)

There are a couple of outdated dropdowns which are used to select timezones. These can be migrated to the existing timezone_dropdown Vue component.

Migrate Timezone Dropdown from custom one to a shared component.

Related to #369974 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Open a project -> CI/CD -> Schedules
2. Create a new pipeline schedule (http://localhost:3000/test-group/test-project/-/pipeline_schedules/new)
3. Observe Timezone Dropdown Cron Timezone
4. Edit the pipeline and choose a new timezone, the update part should work as before

MR acceptance checklist

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