Merge branch '493768-sharding-key-not-null-merge_request_context_commits' into 'master'

Add NOT NULL for sharding key on merge_request_context_commits

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/177405



Merged-by: Jarka Košanová <jarka@gitlab.com>
Approved-by: Shubham Kumar <shukumar@gitlab.com>
Approved-by: Jarka Košanová <jarka@gitlab.com>
Co-authored-by: Shane Maglangit <smaglangit@gitlab.com>