Wrap merge_data & merge_request into single transaction

What does this MR do and why?

With first MR we introduced dual-write on merge_requests table, and in this MR, we wrap all DB write operations into a single transaction for data consistency.

References

• Issue - https://gitlab.com/gitlab-org/gitlab/-/issues/560933+
• Previous MR - Handle dual writes for remaining merge_data att... (!210454 - merged)

Screenshots or screen recordings

NA

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