Add Rest and GraphQL API to configure scheduled merge

<!-- This template is a great use for issues that are feature::additions or technical tasks for larger issues.--> ### Proposal <!-- Use this section to explain the feature and how it will work. It can be helpful to add technical details, design proposals, and links to related epics or issues. --> <!-- Consider adding related issues and epics to this issue. You can also reference the Feature Proposal Template (https://gitlab.com/gitlab-org/gitlab/-/blob/master/.gitlab/issue_templates/Feature%20proposal%20-%20detailed.md) for additional details to consider adding to this issue. Additionally, as a data oriented organization, when your feature exits planning breakdown, consider adding the `What does success look like, and how can we measure that?` section. --> Expand [Schedule MR to be automatically merged based on date/time](https://gitlab.com/gitlab-org/gitlab/-/issues/14380) feature with: 1. GraphQL mutation to update this field on a Merge Request 2. update existing `Update merge request` [REST API Resource](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/api/merge_requests.rb?ref_type=heads#L644) to accept `mergeafter` as a field. <!-- Label reminders Use the following resources to find the appropriate labels: - Use only one tier label choosing the lowest tier this is intended for - https://gitlab.com/gitlab-org/gitlab/-/labels - https://about.gitlab.com/handbook/product/categories/features/ -->

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