Enable `mail_room`'s structured logging in GitLab

What does this MR do?

As part of #30070 (closed), this PR to allow structured logging was merged upstream, giving far more insight into what mail_room is doing behind the scenes.

This MR enables the changes in our codebase by upgrading the gem and adding the requisite config change.

• omnibus MR: omnibus-gitlab!3719 (merged)
• fluentd MR: gitlab-cookbooks/gitlab_fluentd!96 (merged)

Related to #35108 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

Any gem upgrade can introduce behaviour changes that might have unintended regressions. This will need to be tested to ensure it actually works too :)

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Closes #35108 (closed)