Improve regexp for system note email obfuscation

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/169165



Merged-by: Aaron Huntsman <ahuntsman@gitlab.com>
Approved-by: Sam Figueroa <sfigueroa@gitlab.com>
Approved-by: Ottilia Westerlund <12746880-ottilia_westerlund@users.noreply.gitlab.com>
Approved-by: Aaron Huntsman <ahuntsman@gitlab.com>
Reviewed-by: Marc Saleiko <msaleiko@gitlab.com>
Co-authored-by: Marc Saleiko <marc.saleiko@komadesign.de>