Deprecate namespace in mailroom.yml

What does this MR do?

This MR deprecates namespace in mailroom.yml as part of gitlab-com/gl-infra/scalability#2288 (closed).

Namespace deprecation for gitlab rails is handled in gitlab-org/gitlab!133070 (merged). Both changes needs to be in the same release.

Related issues

gitlab-com/gl-infra/scalability#2288 (closed)

Author 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
• When ready for review, follow the instructions in the "Reviewer Roulette" section of the Danger Bot MR comment, as per the Distribution experimental MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added/updated
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened. Not needed as omnibus uses config/mailroom.yml directly
• Equivalent MR/issue for Gitlab Operator project opened (see Operator documentation on impact of Charts changes)
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.