Draft: Refactor createFlash to use createAlert

What does this MR do and why?

This change updates createFlash to use createAlert, as the way to display flash messages. This is conditioned upon two feature flags:

• create_flash_uses_create_alert_project for the first step of the rollout, so we test on gitlab projects.
• create_flash_uses_create_alert_user for the second step of the rollout, when we gradually rollout to all users

If either is enabled for a given actor, all the createFlash usages will result in a display that uses the new createAlert implementation.

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Related to #339653 (closed)