Changes RackAttack logger to use structured logs

What does this MR do?

RackAttack logger is currently using Rails.logger to log rack attack events. This MR changes the initializer to use a structured log instead.

As an effort of https://gitlab.com/gitlab-org/gitlab-ce/issues/48747

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

Performance and testing

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

To do

• Submit an issue a merge request to the gitlab_fluentd project - gitlab-cookbooks/gitlab_fluentd!76 (merged)
• Update GitLab.com runbooks with new logger information. - gitlab-com/runbooks!1092 (merged)

Closes #54528 (closed)