Fix incorrect feature flag check

What does this MR do?

Closes #201728 (closed)

For the new sign up flow, we're using invisible captcha. They are behind a feature flag invisible_captcha. While we only check for the invisible_captcha feature flag in the frontend we would activate the feature in the controller when the user was part of the new sign up experiment.

Therefore, we'd have checked in the controller the invisible captcha, although we didn't render it in the frontend when invisible_captcha was turned off.

No changelog because it is still behind the feature flag experimental_separate_sign_up_flow

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Does this MR meet the acceptance criteria?

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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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