Require sha on merge api on new groups

What does this MR do and why?

This MR prevents calls to the merge API without a SHA parameter, when require_sha_for_merge setting is enabled somewhere in the settings hierarchy

References

gitlab-org/gitlab#600407+

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Create a new group, and open an MR for a project in the group
• Attempt to merge the MR through the API without the SHA param Expectation: Merge blocked with message SHA must be provided when merging.
• Attempt to merge the MR through the API with the SHA param Expectation: Merge successful with 2xx response

MR acceptance checklist

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