Fix deprecated calls to `stub_feature_flags` with argument `thing`

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What does this MR do and why?

Fix deprecated calls to stub_feature_flags with argument thing

• In the past, there used to be a way to disable a feature flag for a specific actor by passing the thing argument to method #stub_feature_flags; this is no longer supported and should be removed since this commit, see gitlab-community/gitlab@e2843005
• Unfortunately, the documentation was not updated and some tests still included this way of using the #stub_feature_flag

🛠 with ❤ at Siemens

MR acceptance checklist

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MR Checklist (@gerardo-navarro)

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the javascript style guides
• Conforms to the database guides

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