Convert events.author_id to users foreign key to a LFK

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/138215



Merged-by: Manoj M J <mmj@gitlab.com>
Approved-by: Manoj M J <mmj@gitlab.com>
Approved-by: Alper Akgun <aakgun@gitlab.com>
Approved-by: James Nutt <jnutt@gitlab.com>
Approved-by: Nao Hashizume <nhashizume@gitlab.com>
Reviewed-by: Doug Stull <dstull@gitlab.com>
Reviewed-by: Manoj M J <mmj@gitlab.com>
Reviewed-by: Lorena Ciutacu <lciutacu@gitlab.com>
Reviewed-by: James Nutt <jnutt@gitlab.com>
Co-authored-by: Omar Qunsul <oqunsul@gitlab.com>