Add update on-call schedules GraphQL mutation

What does this MR do?

Contributes to #262849

Adds a GraphQL mutation to update on-call schedules.

The implementation is behind oncall_schedules_mvc feature flag

Mutation example

If needed follow the steps to create and query an existing on-call schedule.

mutation($oncallScheduleUpdateInput: OncallScheduleUpdateInput!) {
  oncallScheduleUpdate(input: $oncallScheduleUpdateInput) {
    clientMutationId
    errors
    oncallSchedule {
      iid
      name
      description
      timezone
    }
  }
}

query variables

{
  "oncallScheduleUpdateInput": {
    "projectPath": "<PROJECT-FULL-PATH>",
    "iid": "<RECORD-IID>",
    "name": "Updated on-call schedule",
    "description": "Updated on-call schedule description",
    "timezone": "Europe/Lisbon"
  }
}

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry behind a feature flag
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team