Upgrade MailRoom to v0.0.18

What does this MR do?

This adds JWT authentication to support delivering e-mail via an API handler.

Diff: https://my.diffend.io/gems/gitlab-mail_room/0.0.15/0.0.18

Related issues

gitlab-com/gl-infra/scalability#1457 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened