Get rid of the 'brakeman' gem

What does this MR do?

1. We don't use Brakeman except for the brakeman rake task (lib/tasks/brakeman.rake), which I think no one is actually using since we have a brakeman-sast job in our pipelines: .gitlab/ci/reports.gitlab-ci.yml:brakeman-sast.
2. This also removes the gitlab:test and test tasks entirely which I believe no one would ever run considering the duration of these tasks (especially rubocop and spec).

We already advise to run specific commands to run tests: https://docs.gitlab.com/ee/development/rake_tasks.html#run-tests

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
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• Documentation (if required)
• Code review guidelines
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• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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

Related to #327456 (closed)