Use push method on Auditable module

What does this MR do?

Use push method on Auditable module

External collaborators to Gitlab::Audit::EventQueue should not know about the internals of the queue. Hence, we should use a public method, instead of interacting with the array using <<.

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Conformity

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  • I have not included a changelog entry because it is only development-facing change.
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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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Related to #329752 (closed)