The `YAML.safe_load` changes made by
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/58042
broke the diagnostic rake task gitlab:incoming_email:check
due to use of symbols for keys inside the ERB template
file `config/mail_room.yml`.

This change adds an exclusion permitting loading of
Symbol classes when parsing the YAML syntax for this
specific file.

Fixes https://gitlab.com/gitlab-org/gitlab/-/issues/331683

Changelog: fixed