Update Sentry Gems to '~> 5.21.0'

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/168376



Merged-by: Marius Bobin <mbobin@gitlab.com>
Approved-by: Marius Bobin <mbobin@gitlab.com>
Reviewed-by: Peter Leitzen <pleitzen@gitlab.com>
Co-authored-by: GitLab Renovate Bot <gitlab-bot@gitlab.com>