Set continuous onboarding rollout strategy to user

What does this MR do?

Set continuous onboarding rollout strategy to user

After some data flow I detected an issue in record_experiment_user and [push_frontend_experiment](https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/controllers/projects/learn_gitlab_controller.rb#L10) - we have to set the rollout strategy to :user and not :cookie - which we have missed. Fixing it now !62392 (merged)

"# - rollout_strategy: default is :cookie based rollout. We may also set it to :user based rollout" https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/controllers/projects/learn_gitlab_controller.rb#L10

Does this MR meet the acceptance criteria?

Conformity

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Security

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• 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 #281024