Adjust default_enabled to true for wiki_content_background_job flag

What does this MR do and why?

#367628 (closed) was released under a gitlab_com_derisk feature flag. As integrations and webhooks are heavily used, we want to have a longer lived feature.

We need to move from gitlab_com_derisk to beta as gitlab_com_derisk does not allow default_enabled: true.

MR acceptance checklist

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. In rails console enable the experiment fully

   Feature.enabled?(:wiki_content_background_job)

2. End to end test is the same with this steps

Related to #452495 (closed)