Push iteration_cadences frontend FF for direct descendant groups

An iteration cadence could be created for a parent group but would be unavailable for a descendant group. For example, a cadence and iterations could be scoped to gitlab-org but the team that use the cadence could work under a subgroup like gitlab-org/plan-stage. Because we only push :iteration_cadences FF for a group if the group has the FF directly enabled, the group's subgroup cannot use the cadence related features on the frontend.

Discovered the issue in !77755 (comment 807007597).

Suggested fix:

• Check if a group's ancestors have :iteration_cadences FF enabled and push the FF to the frontend accordingly.
• Cache the above result for performance.

Steps to reproduce:

• in a group, create an iteration cadence.
• Then create a subgroup.
• Inside that subgroup, create a project.
• In the project, create an issue.
• Assign the issue to an iteration in the cadence you created.
• You’ll see in the iteration dropdown in the issue sidebar that the iterations are not grouped by their cadence name.
• This is because the FF is not enabled for the Project or the immediate project’s ancestor subgroup