Support accepting epoch millis for alert manager ended at

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/145502



Merged-by: Sashi Kumar Kumaresan <skumar@gitlab.com>
Approved-by: Sashi Kumar Kumaresan <skumar@gitlab.com>
Reviewed-by: Sean Arnold <sarnold@gitlab.com>
Reviewed-by: Sashi Kumar Kumaresan <skumar@gitlab.com>
Co-authored-by: Adam Hegyi <ahegyi@gitlab.com>